Bonding members with epoxy composition containing chain extender, toughener and catalyst

ABSTRACT

A method of assembling a structure comprises applying an epoxy composition to at least one of a first member and a second member, sandwiching the epoxy composition between the first and second members, and bonding the two members wherein the adhesive bond is a thermally cured mass. The epoxy composition contains an epoxy resin, a phenolic or amine compound as a chain extender, a basic catalyst and a polymeric toughener. The composition can be formulated in two parts wherein part A contains the catalyst and part B has the epoxy resin.

The present application is a divisional of U.S. patent application Ser.No. 09/170,597, filed on Oct. 13, 1998 now U.S. Pat. No. 6,486,256.

FIELD OF THE INVENTION

The invention relates to epoxy resin compositions, particularly, to anepoxy resin composition that when cured exhibits properties useful instructural assembly and, even more particularly, to two-part epoxyadhesive compositions that exhibit one or more improved adhesiveproperties such as impact, creep and fatigue resistance, as well asdurability under service conditions for structural applications.

BACKGROUND OF THE INVENTION

Adhesives have been used in many structural applications. Suchstructural applications have included vehicles, computer cases,buildings, appliances, etc. For example, structural adhesives have beenused in vehicle assembly (e.g., automobile and aircraft assembly) toreplace or augment conventional joining techniques such as welds, nutsand bolts, and rivets.

Epoxy compositions are known and have been used for structural adhesiveapplications. In state-of-the-art epoxy technology today, polymerizationcatalysts used to achieve higher order oligomers typically aretetraalkyl ammonium or phosphonium salts that do not promote epoxyhomopolymerization. Cyclic amidine catalysts, such as imidazolinecatalysts and imidazole catalysts, have also been used in adhesives. Theadhesives of the prior art are quality adhesives in many applications.Even so, there is a continuing need for higher performance adhesives inorder to meet the changing needs of various industries such as, forexample, the vehicle assembly industry.

SUMMARY OF THE INVENTION

The present invention is intended, at least in part, to address theongoing need for higher performance adhesives to meet the needs ofvarious industries, including the vehicle assembly industry (e.g.,automobile, aircraft and watercraft industry). Compositions of theinvention are believed to be useful in structural adhesive applicationseither alone or in conjunction with conventional assembly techniqueslike welding and/or mechanical fastening (e.g., rivets).

We have found a composition useful as a structural adhesive having longterm durability under static and/or dynamic loads and substantiallyimproved impact, creep and/or fatigue resistance for use in assemblyapplications. The composition can include a chain extender, a catalyst,a reactive epoxy resin and one or more polymeric tougheners. At leastwhen mixed together, the present adhesive composition is in a form thatcan be applied or dispensed (e.g., liquid or paste form). The chainextender, the reactive epoxy resin, the catalyst and the toughener areeach in an amount and of a type that are effective, when mixed together,to form a thermally curable adhesive. When the adhesive is cured, atleast about 50% by weight of the epoxy resin is chain extended.Preferably, at least 60 wt %, and even more preferably at least 70 wt %,of the epoxy resin is chain extended.

It is preferred that the composition be free, or at least substantiallyfree, of a polyfunctional curing agent (i.e., an agent that chainextends and cross links the epoxy resin). That is, the amount ofpolyfunctional curing agent should be limited to the point that, whenthe adhesive is cured, the desired amount of the epoxy resin is chainextended.

The chain extender can comprise an amine, a phenolic compound or acombination thereof. Preferably, the chain extender is all or at leastsubstantially in monomeric form (i.e., the chain extender is notprereacted, prepolymerized or in oligomeric form) prior to being addedto the composition. That is, enough of the chain extender is inmonomeric form to enable the resulting composition to be applied ordispensed. Preferably, the resulting composition is compatible withstate-of-the-art dispensing and rheology (e.g., viscosity) requirements.It is also preferable that the chain extender be dissolvable into theepoxy resin, the catalyst or both, at least at an elevated temperature(e.g., the curing temperature of the composition). It may be desirablefor the chain extender to be in solid particulate form and finelydispersed in the epoxy resin and/or the catalyst, where the chainextender dissolves at elevated temperatures.

The phenolic compound preferably includes a dihydric phenol (e.g., adi-hydroxy benzene, such as catechol, resorcinol and/or compounds basedthereon), and the amine preferably includes a primary monoamine (e.g.,attached to a primary or secondary carbon), a secondary diamine, andcompounds based thereon. A polyfunctional or multifunctional amine(e.g., a diamine containing both primary and secondary functionality ormultiple primary functionality) will cause chain extending and crosslinking (i.e., will function as a curing agent). Even though it willcause cross linking to occur, a polyfunctional amine or other curingagent may be used, but in a limited amount.

The present composition can be a two-part adhesive with the catalyst ina part A and the reactive epoxy resin in a part B. The chain extender isincluded in at least one of the two parts. When the chain extender ofsuch a two-part adhesive composition includes an amine, the amine ispreferably only in the part A. It may be possible to add very smallamounts of amine in the epoxy part B, as long as the amount of amine isnot enough to adversely affect the part B (e.g., its rheology). When thechain extender of such a two-part composition includes a catechol, thecatechol can be in the part A, in the part B or in both. It ispreferable that the catechol is in at least the part A. It is surprisingthat the catechol can be sufficiently stable (i.e., not recrystalize orreact) in the epoxy resin to provide a part B with a commerciallyacceptable shelf life. When the chain extender includes a catechol andresorcinol, at least the part A includes the resorcinol and catechol.The part B can include the catechol without resorcinol. When the chainextender includes another type of phenolic compound, it may also beincluded in the part A, part B or both.

It can be preferable for at least about 50 wt % of the chain extender tobe catechol. When such a chain extender also includes resorcinol, up toabout 50 wt % of the chain extender can be resorcinol. It is believedthat the adhesive composition can contain in the range of from about 8wt % to about 30 wt % of such a catechol and resorcinol containing chainextender, based on the amount of chain extender and reactive epoxy.

The catalyst is preferably a base. The catalyst can include a cyclicamidine, a tertiary amine, and substituted analogues thereof. Thecatalyst can comprise one or more of imidazole, imidazoline, asubstituted imidazole compound, a substituted imidazoline compound,1,4,5,6-tetrahydropyrimidine, a substituted 1,4,5,6-tetrahydropyrimidinecompound and combinations thereof. The chain extender preferablyincludes catechol. The catalyst can also include one or more substitutedpyridines, pyrrolidines and piperidines. The chosen catalyst orcatalysts should not contain a group that exhibits an electronwithdrawing or stereo chemical effect sufficient to prevent thecomposition, when mixed together, from forming a thermally curableadhesive suitable for structural bonding. Typically, as the mass of thecatalyst increases, the amount of catalyst needed to establish a desiredeffect also increases, unless any substitution chemistry present has anaffect on (i.e., increases or decreases) the effectiveness of thecatalyst. The catalyst can comprise two or more different catalysts. Wehave surprisingly found that a combination of two different amidinecatalyst species, in particular cyclic amidine catalysts, can provideenhanced adhesive properties. A preferred combination can include one ormore imidazole compounds (substituted or unsubstituted) and one or moreimidazoline compounds (substituted or unsubstituted). It is believedthat a combination of a 1,4,5,6-tetrahydropyrimidine compound(substituted or unsubstituted) with an imidazoline compound and/or animidazole compound may also provide enhance adhesive properties.

Preferably, the amount of the catalyst in the adhesive composition is ata level of at least about 0.5 wt-%, more preferably, in the range offrom about 0.5 wt-% to about 10 wt-% or, even more preferably, in therange of from about 0.5 wt-% to about 7.5 wt-%, based on the totalamount of the reactive species or components of the adhesive mass (i.e.,the chain extender, epoxy resin and catalyst) and the molecular weightof the catalyst.

The reactive epoxy resin can comprise one or more glycidyl ether epoxycompounds, each having more than one reactive epoxy group. Preferably,the reactive epoxy resin comprises at least one of an aromatic glycidylether epoxy compound and an aliphatic glycidyl ether epoxy compound,with at least one compound having more than one reactive epoxy group.Typically, the reactive epoxy resin materials are present in amounts inthe range of from about 50 wt-% to about 90 wt-%, and preferably about80 wt-%, based on the reactive species of the composition (i.e.,catalyst, chain extender and epoxy).

It is desirable for the adhesive composition to contain up to 35 parts,preferably in the range of from about 5 parts to about 35 parts, andmore preferably from about 10 parts to about 30 parts, by weight ofpolymeric toughener based on 100 parts by weight of the reactive epoxyresin. For a two-part adhesive composition of the present invention, thetoughener can be added to the part A, the part B or both.

The present adhesives may be used to supplement or completely eliminatea weld or mechanical fastener by applying an adhesive mass between twoparts to be joined and curing the adhesive to form a bonded joint.Optionally, spot welding can be used to pin the parts together until theadhesive is sufficiently cured for handling. Welding can contribute tothe curing process. The adhesives may be used to form assembledstructures by applying the adhesives to augment or replace welded jointsand other mechanical joints. Replacing or supplementing welded jointswith an adhesive bond to create a load bearing joint is believed torequire superior adhesive toughness over a broad temperature range insome applications, as well as adequate adhesion to the substrates beingbonded. This is related directly to the degree of polymeric matrixductility, which requires chain extension allowing for optimumtoughening. Compatibility with state-of-the-art dispensing and rheology(e.g., viscosity) requirements for a flowable one- or two-part adhesivecomposition can require this chain extension to occur, at leastsubstantially if not completely, after the adhesive is applied.

For the purposes of this patent application, the term “active hydrogen”denotes a hydrogen atom in a chemical group wherein the group becomeschemically reactive with the oxirane group resulting in ring openingbonding to the group. Active hydrogens typically are found in amines,thiols, carboxylic acids and phenolics. Preferred active hydrogen groupsinclude amine (—NH—, —NH₂) groups and aromatic hydroxyl (—OH) groups.The function of the active hydrogen compound is to provide chainextension. Some cross-linking can be introduced by polyfunctional aminesbut only to a limited extent. If excessive cross-linking occurs theadhesive can lose toughness and adhesion. Conversely excessive chainextension with little epoxy homopolymerization will result in a weakadhesive.

The first part or part A of the two-part epoxy adhesive comprises thecatalyst. The second part or part B comprises the reactive epoxy portionand optional toughener. One formulation places dihydroxy phenolic in theepoxy part B with only the catalyst in a part A. A second formulationplaces the amine and/or dihydric phenol along with the catalyst in thepart A and the epoxy and a toughener in the part B. A third formulationplaces a portion of the phenolic in both the part A (catalyst) and partB (epoxy).

In the adhesive of the invention, we have found that the stoichiometricequivalents ratio of reactive hydrogen sites to reactive epoxy sites ispreferably less than 1.0 (i.e., for each epoxy equivalent in theadhesive, there is less than 1.0 equivalents of active hydrogen). Wehave also found that it can be preferable for the stoichiometricequivalents ratio to be in the range of from about 0.5 to less than 1.0,in the range of from about 0.6 to less than 1.0, or in the range of fromabout 0.7 to less than 1.0. The active hydrogen sites can be provided bythe chain extender and catalyst. Fillers or the toughener can beindependently incorporated in either or both parts A or B. We have foundthat an amine of the type described above can replace a portion of thedihydric phenol without a loss in physical properties and may be usefulas the only chain extender in the composition. The amine can act as adiluent for the Part A, to lower its viscosity, but may also shorten thework-life of the mixed adhesive. Another function of the amine is toreduce any tendency of the phenolic to recrystallize and help stabilizethe viscosity of the Part A. A further function is providing latitude informulating for a specific volumetric mix ratio to meet dispensingrequirements.

Adhesives made using the formulations of this invention can obtain animpact peel strength of at least about 3 Joules, preferably at leastabout 5 Joules and most preferably at least about 10 Joules at atemperature in the range of from about −40° C. to about 90° C. Thedesirable impact peel strength depends, at least in part, on the type ofsubstrates being adhered together. Further, adhesives according to thepresent invention can form adhesive bonds having a T-peel strength ofgreater than about 70 N/cm width at 23° C., greater than about 85 N/cmwidth at 23° C., and greater than about 100 N/cm width at 23° C. Theadhesive can sustain a load under certain accelerated environmentalcycling conditions for at least 10 days, preferably greater than 20days, most preferably greater than 30 days.

In another aspect of the present invention, a structure is provided thathas a first surface and a second surface joined by an adhesive bond madewith a cured mass of the above described adhesive composition. Thestructural adhesives of the invention can form high quality adhesivebonds between metallic components (e.g., iron, aluminum, titanium,magnesium, copper, etc. and alloys thereof), between non-metallicsubstrates (e.g., reinforced and unreinforced thermoplastic andthermoset polymers, as well as other organic materials or organiccomposite materials) and between metallic and non-metallic substrates.The structure being bonded can form at least a portion of a vehicle.

We have also found that adhesives used to augment or replace weldconstruction can provide useful properties to an assembly. Weldedjoints, while strong, tend to concentrate stress at the weld nuggetperimeter and can fail at the weld perimeter if sufficient impact energyis applied to the joint. Additionally, corrosion resistance associatedwith the weld nugget and adjacent metal is typically reduced. Curedepoxy adhesives of the present invention can absorb substantial impactenergy and dissipate the energy throughout the structure. Such energydissipation properties, in conjunction with weld joints, can improve thesurvivability of a structure under conditions of high impact loads. Suchan adhesive requires significant structural properties. Regardless ofthe direction of the impact energy, it may be desirable for the adhesivemass to be able to maintain structural integrity regardless of whetherthe adhesive is exposed to stress in a cleavage mode, a shear mode, acompression mode or a tensile mode. Therefore, the structure cancomprise a joint having a welded bond in addition to the adhesive bond.In addition, the welded bond can be formed through the adhesive bond.

We have found two characteristics of adhesives that can help identify anadhesive that is useful in this type of application. We have found thatthe impact peel strength and T-peel adhesion of the adhesive can beuseful indicators for adhesive utility. Other characteristics ofadhesives useful as performance indicators can also include sustainedload durability and fatigue resistance. The epoxy adhesives of theinvention may be used in a structure having structural integrity that ismaintained with both welded joints and adhesive bonds made using thecurable adhesive of the invention or with only such adhesive bonds.Adhesives are also desirable, for example in the automotive industry,because in an effort to reduce weight, car manufacturers are looking touse thinner gauge steel either alone or in combination with aluminum,magnesium, etc. In addition, the present adhesives can be a viableoption for bonding together various organic materials or compositeswhich cannot be welded or joined with conventional methods. Additionalbenefits of a structure bonded according to the invention are believedto include improved crash worthiness (i.e., impact resistance),survivability, corrosion resistance, sealing of the joint and vibrationdamping.

In an additional aspect of the present invention, a method is providedfor assembling the above described adhesively bonded structure. Themethod comprises the steps of: (a) applying an uncured mass of thecomposition of claim 1 to at least one of a first member and a secondmember; (b) sandwiching the uncured mass between the first member andthe second member; and (c) curing the composition to form an adhesivebond so as to adhere the first member and the second member together.The first member can be a frame member and the second member can be asheet-like member or another frame member. The method can also includethe step of welding the sheet-like member to the frame member throughthe uncured mass before the curing step.

Adhesives can be used for such applications, but known structuraladhesives that are currently available for such applications do not havethe requisite combination of properties and performance over typical enduse (e.g. service) temperature ranges. Such properties include long termdurability and fatigue resistance under static and dynamic (e.g. cyclic)loads and good impact resistance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-5 show the test device and test specimen used to determineimpact peel strength. FIGS. 1 and 2 show the test wedge used in thetest. FIGS. 3-5 show the test specimen configured for the test, itsinstallation on the wedge and the specimen after application of the testforce.

FIG. 6 shows an assembly comprising a hydroformed tube structurallyassembled with and adhered to a panel using the adhesive of theinvention. This assembly structure includes means that maintain thestructure and position during adhesive curing.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

The two part epoxy adhesive compositions of the present inventioncomprise a catalyst part A (e.g., a cyclic amidine) and an epoxy part B.Either the part A, part B or both will also contain a chain extender anda toughener. The novel compositions of the invention provide ductile andtough structural adhesives that have good long term durability andfatigue resistance under static loads and dynamic loads. The resincompositions of the invention are believed especially useful asstructural adhesives where the operating temperature of the bondedarticle or material is expected to be substantially above and/or belowroom temperature, such as the range of service temperatures typicallyseen by a vehicle (e.g., automobiles, aircraft and watercraft). Thepresent epoxy adhesives, when cured, are believed be useful attemperatures in the range of from about −40° C. to about 90° C. and,more desirably, from about −40° C. to about 120° C. Additionally, whenused to join together parts of the frames of vehicle bodies, theadhesives can stiffen the joints and thus stiffen the vehicle'sstructure, as well as impart sealing and/or vibration damping propertiesto the vehicle bodies.

Preferred adhesive compositions of the present invention can comprise aglycidyl ether type epoxy, an amine (e.g., a primary monoamine and/orsecondary diamine) and/or a dihydric phenol, a cyclic amidine catalyst,and a toughening agent, in which the epoxy and chain extender (i.e.,amine and/or dihydric phenol) are substantially unreacted before thecomposition is exposed to a catalyst to cure the adhesive.

Another embodiment includes a two-part epoxy composition comprising aPart A or first part having a cyclic amidine catalyst, and a Part B orsecond part comprising an epoxy, a dihydric phenol and toughening agentwherein the epoxy and dihydric phenol are substantially unreacted, and atwo-part epoxy composition comprising Part A having a cyclic amidinecatalyst and catechol (i.e., 1,2-dihydroxybenezene), and a Part Bcomprising epoxy and toughening agent where a portion of the dihydricphenol can also be incorporated in Part B. A third embodiment comprisesa Part A having a cyclic amidine catalyst, dihydric phenol and an amine(i.e., a primary monoamine and/or secondary diamine), and a Part Bcomprising epoxy and toughener. In all of these compositions fillers canbe incorporated in both parts A and B. The amount (wt-%) of Part A andPart B can be substantially varied by the amount of filler used and thecomposition of each Part.

The two part epoxy adhesive compositions of the invention containdihydric phenol. Suitable dihydric phenolic compounds of the inventioninclude bisphenols (e.g., Bisphenol A, Bisphenol F, etc.)dihydroxynaphthalenes and dihydroxybenzenes. These dihydric phenoliccompounds may be substituted or non-substituted. For example, suitablebisphenols and dihydroxybenzenes may include those that are alkyl,halogen or alkoxy substituted. Suitable dihydroxybenzenes arerepresented by the following formula:

wherein the hydroxyl groups can be ortho or meta on the aromatic ringand R represents one, two or more typical substituents. In the aboveformula R represents any useful ring substituent including hydrogen.Included in these categories are 1,2 dihydroxybenzene (catechol), 1,2dihydroxy-4-methyl-benzene, 4-t-butylcatechol, and 1,3-dihydroxybenzene(resorcinol), 3-methoxy-catechol and others. It is believed undesirableto have bulky substituents that can cause significant steric hindranceadjacent to the phenolic hydroxyls.

Preferred dihydric phenols include those which have hydroxyl groupsattached to adjacent carbon atoms on the aromatic ring, and substitutedanalogues of these compounds. Preferred compounds include catechol,3-methoxycatechol, 3-methylcatechol, 3-fluorocatechol, 4-methylcatecholand blends thereof. Preferably, the dihydric phenol is catechol, or ablend of catechol and one or more other dihydric phenols. For a blend ofdihydric phenols, satisfactory results have been obtained with thecatechol being present in an amount of at least about 50 percent byweight (wt-%) of the total dihydric phenol amount. We have found thatthese compounds are particularly useful in forming the high structuralstrength two part epoxy adhesives of the invention.

The epoxides that are useful in the composition of the present inventionare of the glycidyl ether type. Preferred epoxides include glycidylethers of Bisphenol A and F; aliphatic or cycloaliphatic diols. Usefulmaterials can include those having a molecular weight in the range offrom about 170 to about 10,000, and preferably from about 200 to about3,000. Useful materials can include linear polymeric epoxides havingterminal epoxy groups (e.g., a diglycidyl ether of a polyoxyalkyleneglycol. Useful epoxides can include those having the general formula:

wherein: R′ is alkyl, alkyl ether, or aryl, preferably aryl, and n isgreater than 1 or in the range of from 1 to 4. Aromatic glydicyl etherscan be preferred, such as those prepared by reacting a dihydric phenolwith an excess of epichlorohydrin. Examples of useful dihydric phenolsinclude resorcinol, catechol, hydroquinone, and the polynuclear phenolsincluding p,p′-dihydroxydibenzyl, p,p′-dihydroxydiphenyl,p,p′-dihydroxyphenyl sulfone, p,p′-dihydroxybenzophenone,2,2′-dihydroxy-1,1-dinaphthylmethane, and the 2,2′, 2,3′, 2,4′, 3,3′,3,4′, and 4,4′ isomers of dihydroxydiphenylmethane,dihydroxydiphenyldimethylmethane, dihydroxydiphenylethylmethylmethane,dihydroxydiphenylmethylpropylmethane,dihydroxydiphenylethylphenylmethane,dihydroxydiphenylpropylphenylmethane,dihydroxydiphenylbutylphenylmethane, dihydroxydiphenyltolylethane,dihydroxydiphenyltolylmethylmethane,dihydroxydiphenyldicyclohexylmethane, and dihydroxydiphenylcyclohexane.Examples of commercially available aromatic and aliphatic epoxidesuseful in the invention include diglycidyl ethers of bisphenol A (e.g.,those available under the trademarks Epon 828, Epon 1001, Epon 1310 andEpon 1510 from Shell Chemical Co., and DER-331,DER-332, and DER-334available from Dow Chemical Co.); diglycidyl ethers of bisphenol F(e.g., Epiclon TM830 available from Dainippon Ink and Chemicals, Inc.);silicone resins containing diglycidyl epoxy functionality; flameretardant epoxy resins (e.g., DER 580, a brominated bisphenol type epoxyresin available from Dow Chemical Co.); 1,4-dimethanol cyclohexyldiglycidyl ether and 1,4-butanediol diglycidyl ethers. In some casesreactive diluents may be added to control the flow characteristics ofthe adhesive composition. Suitable diluents can have at least onereactive terminal end portion and, preferably, a saturated orunsaturated cyclic backbone. Preferred reactive terminal ether portionsinclude glycidyl ether. Examples of suitable diluents include thediglycidyl ether of resorcinol, diglycidyl ether of cyclohexanedimethanol, diglycidyl ether of neopentyl glycol, triglycidyl ether oftrimethylolpropane. A commercially available reactive diluent isReactive Diluent “107” from Shell Chemical Company.

The components of the composition can be present in amounts such thatthe stoichiometric equivalents ratio of reactive hydrogen sites toreactive epoxy sites is less than 1.0, in the range of from about 0.5 toless than 1.0, from about 0.6 to less than 1.0, and from about 0.7 toless than 1.0. The equivalents ratio is defined as the number ofequivalents of reactive hydrogen sites divided by the number ofequivalents of reactive epoxide sites. The active hydrogen sites caninclude the chain extender phenolic —OH, the chain extender amine —NH or—NH2, the catalyst amine —NH or combinations thereof.

We have found that the present epoxy compositions can be catalyzed withan amidine catalyst or a blend of two or more different amidinecatalysts. The preferred catalysts include cyclic amidines (e.g., animidazole, imidazoline and 1,4,5,6-tetrahydropyrimidine) and substitutedanalogs of cyclic amidines. An amidine is generally defined as the group—N═C—N—. Suitable ring substituents for a cyclic amidine catalyst caninclude a substituent such as methyl, ethyl, isopropyl, cyanoethyl,acetyl, carboxamide, methylol, etc. (e.g., for an imidazole, imidazolineor 1,4,5,6-tetrahydropyrimidine compound). The secondary nitrogen onthese catalyst compounds can be further substituted as well (e.g.,1-acetylimidazole).

The preferred catalysts can include substituted imidazolines,substituted 1,4,5,6-tetrahydropyrimidines, and blends of one or both ofan imidazoline and a 1,4,5,6-tetrahydropyrimidine with another amidinecatalyst (e.g., imidazole or a substituted imidazole). Preferably, atleast the imidazoline and the imidazole catalysts do not contain anelectron withdrawing group (e.g., phenyl, nitro, carbonyl or halogen) ontheir respective ring. It is believed preferable for the ring of1,4,5,6-tetrahydropyrimidine and other amidine catalysts to also be freeof an electron withdrawing group. Even so, some degree of electronwithdrawing can be acceptable in certain positions on the ring.Preferred substituents can include aliphatic groups in the 2-positionsuch as the following 2-ethyl-4-methyl-imidazoline:

Other preferred substituents can include aliphatic groups in the 1- and2-positions such as 1,2-dimethyl-1,4,5,6-tetrahydropyrimidine.

In the preferred practice of the invention, the amount of catalyst orblend thereof, is selected to provide a cured epoxy adhesive havingunexpected properties such as sustained load durability and impactresistance, preferably over a wide range of temperatures. It has beenfound that the amount of catalyst used can provide the necessary balanceof epoxy homopolymerization and copolymerization with amine and/ordihydric phenol (e.g., catechol) to provide the properties needed forboth low and high temperature performance. The preferred amounts ofcatalyst can vary depending upon the catalyst type and activehydrogen/epoxy ratio (NH,OH/Epoxy ratio). The useful range needs to behigh enough to effect both the epoxy copolymerization andhomopolymerization reaction. A level of catalyst too low or too highwill result in a weak adhesive leading to poor performance. The optimumamount of catalyst can also vary with the catalyst chemistry.

For a preferred catalyst chemistry in the case of:2-ethyl-4-methylimidazoline the range can be from about 1.0% to about8.0% by weight (wt-%) based on the weight of epoxy, chain extender(i.e., amine and/or catechol) and catalyst, and preferably from about 2wt-% to about 7.0 wt-%; 2-benzyl-2-imidazoline the range can be fromabout 3.0 wt-% to about 11.0 wt-% and preferably from about 4.0 wt-% toabout 10.0 wt-%; 4,4′-dimethyl-2-imidazoline the range can be from about3.0 wt-% to about 7.0 wt-% and preferably from about 4.0 wt-% to about6.0 wt-%. Other preferred catalysts can include: imidazole in the rangeof from about 0.25 wt-% to about 3.0 wt-% and preferably from about 0.5wt-% to about 2.25 wt-%; DBUE(1,4-Diazabicycol<5.4.0>undec-7-ene) in therange of from about 4.0 wt-% to about 8.0 wt-% and preferably from about5.0 wt-% to about 7.0 wt-%; 1-butylpyrrolidine in the range of fromabout 3.0 wt-% to about 7.0 wt-%; 1,4,5,6-tetrahydropyrimidine in therange of from about 3.0 wt % to about 8.5 wt %;1,2-dimethyl-1,4,5,6-tetrahydropyrimidine in the range of from about 1.5wt-% to about 6.0 wt-%; and N,N-dimethylbenzylamine in the range of fromabout 4.0 wt-% to about 8.0 wt-%.

Toughening agents (or elastomeric modifiers) for use in preferredcompositions of the present invention generally comprise: polymericcompounds having both a rubbery phase and a thermoplastic phase such asgraft copolymers having a polymerized diene rubbery core and apolyacrylate or polymethacrylate shell; graft copolymers having arubbery core with a polyacrylate or polymethacrylate shell; andelastomeric particles polymerized in situ in the epoxide fromfree-radical polymerizable monomers and a copolymeric stabilizer;elastomer molecules, separate elastomer precursor molecules; combinationmolecules that include epoxy-resin segments and elastomeric segments;and, mixtures of such separate and combination molecules. Thesematerials are used to improve structural properties including peelstrength. The combination molecules may be prepared by reacting epoxyresin materials with elastomeric segments; the reaction leaving reactivefunctional groups, such as unreacted epoxy groups, on the reactionproduct. The general use of tougheners in epoxy resins is well-known,and is described in the Advances in Chemistry Series No. 208 entitled“Rubbery-Modified Thermoset Resins”, edited by C. K. Riew and J. K.Gillham, American Chemical Society, Washington, 1984, the referencebeing incorporated herein by reference. The amount of toughening agentto be used depends in part upon the final physical characteristics ofthe cured resin desired, and is generally determined empirically.

Specific examples of useful toughening agents include graft copolymershaving a polymerized diene rubbery backbone or core to which is grafteda shell of an acrylic acid ester or methacrylic acid ester, monovinylaromatic hydrocarbon, or a mixture thereof, such as disclosed in U.S.Pat. No. 3,496,250, incorporated herein by reference. Preferable rubberybackbones can comprise polymerized butadiene or a polymerized mixture ofbutadiene and styrene. Preferable shells comprising polymerizedmethacrylic acid esters can be lower alkyl (C₁₋₄) substitutedmethacrylates. Preferable monovinyl aromatic hydrocarbons can bestyrene, alpha-methylstyrene, vinyltoluene, vinylxylene,ethylvinylbenzene, isopropylstyrene, chlorostyrene, dichlorostyrene, andethylchlorostyrene.

Further examples of useful toughening agents are acrylate core-shellgraft copolymers wherein the core or backbone is a polyacrylate polymerhaving a glass transition temperature T(g) below about 0° C., such aspolybutyl acrylate or polyisooctyl acrylate to which is grafted apolymethacrylate polymer (shell) having a T(g) about 25° C. such aspolymethylmethacrylate.

Still further examples of toughening agents useful in the invention areelastomeric particles that have a T(g) below about 25° C. and have beenpolymerized in situ in the epoxide before mixing with the othercomponents of the composition. These elastomeric particles arepolymerized from free-radical polymerizable monomers and acopolymerizable polymeric stabilizer that is soluble in the epoxide. Thefree-radical polymerizable monomers are ethylenically unsaturatedmonomers or diisocyanates combined with coreactive difunctional hydrogencompounds such as diols, diamines, and alkanolamines. Examples of theseelastomeric particles are disclosed in U.S. Pat. No. 4,525,181, which isincorporated herein by reference. These particles are commonly referredto as “organosols”.

Still other toughening agents are rubber modified liquid epoxy resins.An example of such a resin is Kraton™ RP6565 Rubber available from ShellChemical Company. The modified epoxy resin is made from 85% by weightEpon™ 828 and 15% by weight of a Kraton™ rubber. The Kraton™ rubbers areknown in the industry as elastomeric block copolymers.

The toughening agent is preferably used in an amount up to about 35parts by weight per 100 parts of epoxy resin. Above 35 parts oftoughening agent, the composition can become very viscous and mayrequire a preheating or prewarming to facilitate its dispensing. Thetoughening agents of the present invention add toughness to thecomposition after curing. Some toughening agents can react and otherswill not react with the epoxide.

Other useful toughening agents include: carboxylated and amineterminated acrylonitrile/butadiene vulcanizable elastomer precursorssuch as Hycar® CTBN 1300X8 and ATBN 1300X16 and Hycar® 1072 from B. F.Goodrich Chemical Co.; butadiene polymer such as Hycar® CTB; aminefunctional polyethers such as HC1101 (i.e., polytetramethylene oxidediamine) a 10,000 MW, primary amine-terminated, compound from MinnesotaMining and Manufacturing Co.; St. Paul, Minn., and Jeffamine® fromHuntsman Chemical Co. in Houston, Tex.; functional acrylic rubbersincluding acrylic core/shell material, such as Acryloid® KM330 and 334from Rohm & Haas; and core/shell polymers, such asmethacrylate-butadiene-styrene (MBS) copolymer wherein the core iscrosslinked styrene/butadiene rubber and the shell is polymethylacrylate(e.g., Acryloid® KM653 and KM680; Rohm and Haas). As used above, foracrylic core/shell materials “core” will be understood to be acrylicpolymer having Tg<0° C. and “shell” will be understood to be an acrylicpolymer having Tg>25° C. Tougheners may include epoxy-terminatedcompounds, which can be incorporated into the polymer backbone. Atypical, preferred, list of tougheners includes: acrylic core/shellpolymers; styrene-butadiene/methacrylate core/shell polymers; polyetherpolymers; carboxylated acrylonitrile/butadienes; and, carboxylatedbutadienes. Advantages can be obtained from the provision of the chainextension agent in a composition with an epoxy resin even in the absenceof a toughening agent as described above. However, particular advantageis achieved from the presence of the toughening agent or combinations ofdifferent agents, as previously suggested. It is a feature of thepresent invention that improved resins as disclosed herein are generallymade particularly susceptible to, or are enhanced with respect to, thebeneficial effects of tougheners.

When included in the epoxy adhesives of the invention, typically in PartA (the catalyst part) the amine or amines used are able to achieve chainextension of the growing polymeric chain during curing. Preferred aminescan have a reactive or active hydrogen functionality of two. Such usefulamines include normally liquid amines compatible in part A alone or incombination with the catechol used in the adhesive of this invention.Useful amines include aliphatic primary monoamines, secondary diaminesand other amines having two reactive hydrogens per molecule. Such aminescan have other reactive hydrogens if they are sterically or otherwisehindered and are substantially non-reactive during curing. Preferredamines are substantially free of electron withdrawing groups in aposition that reduces reactivity of active hydrogens in the amine.Suitable amines can include polyether monoamines, amido mono- anddi-amines, aliphatic primary monoamines, polyether diamines withsecondary nitrogen groups, diamines with secondary nitrogen groups,monoalkanolamine, etc. Preferred amine compounds can include compoundsof the formula:

wherein R, R₁ and R₂ are independently selected from the groupconsisting of aliphatic, aryl (aromatic) or hydrogen; wherein n and mare numbers independently selected from 0 to 3; x is a number thatranges from 0 to 10; and Y can be —O— or —S—. In another amine, possiblyuseful in limited amounts, Y is —NH—.

An additional embodiment of the chain extender amines are compoundsaccording to the formula:

R₁—NH—(CH₂)_(x)—R₃—(CH₂)_(x)—NH—R₂

wherein R₁ and R₂ are independently selected from the group consistingof alkyl, benzyl, —CH₂—CH₂—CN; R₃ is independently selected from thegroup consisting of —CH₂—, —S—, —O—CH₂—CH₂—O—, or arylene structuresincluding phenylene or naphthalene and each x is independently a numberthat ranges from 1 to 3. In another amine, possibly useful in limitedamounts, R₃ is —NH—.

A third embodiment of the chain extender amine compounds are compoundsof the formula:

wherein R₂ is an aliphatic group or an aromatic group, containing 1 to18 carbon atoms and R₁ and R₃ are R₂ or H.

One specific embodiment of the chain extender amine of the presentinvention comprises alkyl amino substituted morpholine (e.g.,4-(3-aminopropyl)morpholine). Another amine, possibly useful in limitedamounts, may be an alkyl amino substituted piperazine. Each of theseamines has the following formula:

wherein Y is —O— for the alkyl amino substituted morpholine and Y is—NH— for the alkyl amino substituted piperazine.

It can be desirable for the above amines to be used in the adhesive atamounts in the range of from about 0.5 wt-% to about 20 wt-%, preferablefrom about 2 wt-% to about 15 wt-%, based on all of the reactivecomponents (i.e., the epoxy resin, catalyst and chain extender). Whencatechol and an amine are used together, it can be desirable for thecatechol/amine weight ratio to be in the range of from about 4 to about0.5 parts of catechol per 1 part by weight of amine, and preferable atleast about 1 part catechol per 1 part by weight of amine.

It has been found that the risk of the catechol recrystallizing can beeffectively eliminated, or at least significantly reduced, by addingsome resorcinol and/or amine as part of the chain extender. The use of aliquid amine can help to prevent recrystallization of the catechol, whenthe catechol is mixed with the amine, depending on the solubility of thecatechol in the amine. The use of a liquid catalyst may also help toprevent recrystallization of the catechol, when the catechol is mixedwith the catalyst, depending on the solubility of the catechol in thecatalyst. Preferably, the catechol is soluble in both the catalyst andthe amine, when an amine is used. Furthermore, it has been found thatthe addition of an amine can provide faster reactivity (i.e., shortercuring times) and greater latitude in the mix ratio between parts A andB. By adding an amine in the chain extender, the adhesive compositioncan gel more quickly to a tack free state. How quickly the compositiongels depends on the amount and type of amine used. If the amineconcentration is too high, it may impact performance of the adhesivecomposition.

Various adjuvants may be added to compositions according to the presentinvention, to alter the characteristics of the cured composition.Included among useful adjuvants are: corrosion inhibitors such as somesilica gels; thixotropic agents such as fumed silica; pigments such asferric oxide, brick dust, carbon black, and titanium oxide; reinforcingagents such as silica, magnesium sulfate, calcium sulfate, and berylliumaluminum silicate; clays such as bentonite; and any suitable filler.Amounts of up to about 50 parts of adjuvant, and possibly more, per 100parts of liquid adhesive components may be effectively utilized.Generally, the toughening agent is pre-dispersed in the epoxidecompound. The toughener-containing epoxide part B is then mixed with acurative part A, with the chain extension agent in the part A, the partB or both parts, to form a substantially uniform mixture.

The mixture is cured upon heating for an appropriate length of time.While partial curing reaction may take place slowly at room temperature,full cure is preferably brought about by heating the mixture to atemperature in the range of from about 130° C. to about 200° C. for anappropriate length of time. A typical heating cycle may be 20 minutes at163° C. Generally, as the curing temperature increases, the curing timedecreases.

The adhesives of the invention may be used, for example, to assemblepanels or other sheet-like structures with frame members. As shown inFIG. 6, the adhesive may be used in combining a panel with a hydroformedtube frame structure using self-positioning means to hold the parts in acorrect alignment while the adhesive cures. Such an assembly system is asubstantial advancement over other systems. In addition, the adhesivemay be useful in bonding together members of a space frame. Furthermore,the adhesive may be used, for example, in an automobile to bond weldpaddles onto an intrusion beam in order to make a door intrusion beamassembly. The adhesive may also be used to adhesively bond the doorintrusion beam assembly in the automobile door. Welding (e.g., tackwelding) or mechanical fastening could be used to fix the adhesivelybonded paddles in place until the adhesive cures. It may also bedesirable to use the adhesive to bond hydroformed tube steel together inorder to make an automobile space frame assembly. Another use for theadhesives of the invention involves hem bonding of two substrates withan appropriate mechanical structure. In hem bonding, an adhesive mass isformed between the edges of two substrates brought into close alignment.The edges of the substrates are bent in an overlapping fashion to form afolded or bent edge structure with the adhesive found between thesubstrates throughout the folded or overlapped edge. The thus formededge structure can then be cured through induction heating or othercommon heat curing methods (e.g., infrared radiation, forced air,immerson, etc.).

In areas of adhesive bonding, the adhesive can be applied as acontinuous bead, in intermediate dots, stripes, diagonals or any othergeometrical form that will conform to forming a useful bond. Suchadhesive placement options can be augmented by welding. The welding canoccur as spot welds, as continuous seam welds, or as any other weldingtechnology that can cooperate with an adhesive mass to form amechanically sound joint that has adequate fatigue and impact resistanceand load bearing performance. Such welding can occur around or throughthe adhesive bonds. The heat of welding can augment other curing energyinputs (e.g., oven baking, induction heating, etc.).

The specification provides an explanation of the components andprocessing used to make and use the epoxy compositions of the invention.The following examples and data further exemplify the invention anddemonstrate the advance in structural adhesives achieved by thisinvention.

Preparation of Substrates

FPL Etched Aluminum Substrate: The aluminum substrate is a 102 mm by 178mm by 0.8 mm thick sheet of 2024T-3 Alclad aluminum obtained from AlcanCorporation. Each sheet or coupon is treated as follows beforetesting: 1) soaking for 10 minutes in Oakite™ 165 caustic wash solution,obtained from Oakite Corp., St. Paul, Minn., at a temperature of 85° C.;2) the sheets (in a rack) are submerged in tank of tap water for 10minutes; 3) spray rinsing with tap water for 2-3 minutes; 4) soaking ina tank of FPL etch (a hot solution of sulfuric acid and sodiumdichromate from Forest Products Laboratory of Madison, Wis.) at 66° C.for 10 minutes; 5) spray rinsing with tap water for 2-3 minutes; 6) dripdrying for 10 minutes at ambient temperature and then for 30 minutes ina re-circulating air oven at 54° C.; 7) spraying a primer (EC3960available from 3M Company, St. Paul, Minn.) to a coating thickness of0.25 to 0.50 mm; 8) drying at ambient temperature (about 23° C.) for 30minutes followed by drying in a re-circulating air oven at about 121° C.for one hour.

Steel Substrate: The steel substrate is a 25 mm by 100 mm by 0.8 mmthick coupon of hot dipped minimum spangled galvanized steel (G60HDMSobtained from National Steel Corporation, Livonia, Mich.) unlessotherwise noted. The steel is cleaned by applying methyl ethyl ketone(MEK) to the surfaces, wiping with cheesecloth, and then drying forabout 10 minutes at room temperature.

Lubricated Steel Substrate: A lubricated steel substrate is prepared bytaking the steel coupon described above that has been cleaned withmethyl ethyl ketone and then applying a controlled coating weight of61MAL automotive lubricant obtained from Quaker Corp., Chicago Ill.,unless otherwise noted. The lubricant is applied with an Eppendorf®Repeater™ Pipette #4780 with a 1 microliter tip. A setting of #4 on thepipette dial was used to dispense 3 drops of lubricant (i.e., 12microliters) onto the cleaned steel surface, and then smeared to an evencoating with a latex gloved finger. The coating weight is measured to beabout 400±50 milligrams per square foot (about 4.3±0.54 g/m²).

Test Methods

Test Method A: T-Peel Adhesion Test on an FPL Etched Aluminum Substrate

The aluminum sheet substrate is prepared as described above for FPLEtched Aluminum. The test adhesive is applied over the entire primedsurface. Glass fibers (diameter 0.13 mm) are then laid across theadhesive at a 45 degree angle at a density of about one fiber every 25.4mm. A second prepared aluminum test substrate is placed over the firstone at a 12.7 mm offset in the lengthwise dimension to facilitateopening of the bond for a T-peel configuration and with the preparedsurface against the adhesive. The sample is then placed between two 203mm by 203 mm by 6.4 mm thick steel plates, put into a press applying27.6 kiloPascals (kPa), and cured at about 121° C. for 60 minutes. Thelaminate is then allowed to equilibrate at 23° C. and 50% relativehumidity (RH) for 24 hours. Test samples measuring 25.4 mm by about 178mm test sample are cut from the sheet and tested for T-peel on anInstron Tensile Tester following ASTM D1876-72 at a crosshead speed of127 mm-min⁻¹. Results are reported in Newtons per centimeter (N-cm⁻¹.

Test Method B: T-Peel Adhesive Strength on Steel or Lubricated SteelSubstrate

The test adhesive is spread over the prepared surface of two steel orlubricated steel coupons described above except for a 15-20 mm sectionleft free of adhesive on the opposite ends of each test strip. Theadhesive contains solid glass beads having a diameter of 0.25 mm±0.01mm, obtained from Cataphote, Inc., Jackson, Miss. The beads are used tocontrol bondline thickness and the adhesive is spread with a spatula byapplying pressure so that the spatula is contacting the glass beads. Thetwo strips are brought together, and clamped with two medium sizedbinder clips along each of the 100 mm edges. The coupons remain clampedtogether and are placed in a forced air oven at 163° C. for 20 minutesto cure the adhesive and form a coupon assembly. The non-adhesivelybonded ends of the coupon assembly are each then pried open to form aT-shaped configuration at either end of the coupon assembly. The couponassembly is then allowed to equilibrate at room temperature. The T-peelstrength is performed according to ASTM Method D 1876-72 using anInstron Tensile Tester at a crosshead speed of 127 mm-min⁻¹. Results arereported in Newtons per centimeter (N-cm⁻¹).

Test Method C: Overlap Shear Test for Aluminum Substrate

Test sheets of aluminum are prepared as described for the T-peel test.The test adhesive is applied over about 12.2 mm of a primed sheet ofaluminum. Glass fibers are applied at a 45 degree angle as describedabove. The primed surface of a second sheet of aluminum is pressed intothe adhesive such that the second sheet overlaps the adhesive 12.7 mmwith the non-adhesive portions of each of the sheets extending inopposite directions. The sample is cured between steel plates asdescribed above, and then conditioned at 23° C. and 50% RH for at least24 hours. Test samples measuring 25.43 mm in width are cut from thecured sample. Overlap shear strength is determined on an Instron TensileTester following ASTM TM D1002-72 at a crosshead speed of 50 mm per min.

Test Method D: Overlap Shear Strength on Cleaned Steel or LubricatedSteel

The test adhesive is applied over 12.72 mm on one end of two 25 mm by100 mm cleaned steel or lubricated steel coupons and spread down to thelevel of glass bead particles contained within the adhesive, asdescribed above for Test Method B. The two adhesive coated ends arepressed together forming a 12.72 mm overlap with the non-adhesive endsof the coupons extending in opposite directions. The overlapped couponsare clamped together at the adhesive ends using a 0.94 cm capacitybinder clip (No. 1002 available from IDL MFG and Sales Corp., Carlstadt,N.J.). The clamped assembly is then cured in a forced air oven at 163°C. for 20 minutes. The laminate is then allowed to equilibrate at roomtemperature. Overlap shear strength is determined according to ASTMD1002-72 with an Instron Tensile Tester at a crosshead speed of 50 mmper minute. Test results are reported in megaPascals (MPa).

Test Method E: Impact Peel Test (Dynamic Wedge Impact)

This test is used to evaluate the relative ability of an adhesivebonding system to dissipate energy in the peel mode during an impactload. The method is a Ford Laboratory Test Method and is an extension toISO Method 11343 with the same specimen size and wedge shape as the ISOmethod.

The wedge shape is shown in FIGS. 1 and 2. It measures 117.3 mm inheight and 20 mm at the base with a radius of 1.0 mm at the tip for anangle of 8.8°, and is fabricated from hardened steel. The impact portionof the test transducer (hammer) is fabricated from hardened steel andmeasures at least 25 mm by 5 mm in thickness to insure impact over theentire top of the test assembly. In FIG. 1, the test wedge shaped 10 isshown having a base 11 secured using drilled and tapped mountingnapatures 12 and the wedge 13. In FIG. 2, a top view of the wedge showedin side view in FIG. 1 is shown having the wedge 13′, the base 11′ andthe tapped and drilled holes 12′.

The test is performed using an instrumented impact testing machinecalled a Dynatup Impact Test Machine, Model 8250 made by Instron Corp.(formerly General Research Corp.) of Canton, Mass. The impact hammer isa force transducer classified as a drop weight “tup”.

The test substrates used are MEK (methyl ethyl ketone)cleaned G60hot-dipped minimum spangled galvanized metal coupons (obtained fromNational Steel Corp.) measuring 20 mm×90 mm×0.78 mm with tolerance onlength and width being ±0.1 mm. Test specimens are prepared by firstaligning two metal coupons so that their 90 mm sides are touching. Then19 mm wide Kapton tape (3M Company tape #5419) is applied to a distanceof 30±0.2 mm from the ends of both coupons across both coupons. The testadhesive is applied to the 30 mm exposed surface at the ends of bothcoupons, as described above for Test Method B. Any adhesive on theKapton tape surface is removed. Both coupons are pressedtogether—adhesive to adhesive—and excess adhesive that has squeezedbeyond the edges is removed. The test assembly is then clamped withmedium size binder clips followed by curing at 163° C. for 20 minutes.The cured test assembly is then allowed to equilibrate at roomtemperature prior to testing. FIG. 4 shows the appropriate configurationwith the coupons 21 and 22 having the test adhesive mass 23 adhering thecoupons.

The test assembly and wedge are maintained at a constant temperaturespecified for the test (23° C. or 90° C., both ±1° C.). The assembly ismarked 40.0±0.2 mm from the bonded end for consistent placement on thewedge. The nonbonded end of the assembly is then slipped over the wedge10 and pushed down until reaching the 40 mm mark. The assembly is notprebent, but allowed to conform to the shape of the wedge. FIG. 4 showsthe appropriate configuration with the coupons 21 and 22 with the testadhesive mass 23 adhering the coupons. The assembly is positioned on thewedge knife edge so that it is square with respect to the wedge andimpact hammer and that the hammer hits the entire top of the testassembly simultaneously. The test machine is activated by impacting thespecimen with a falling weight 31 of 44.8 kg at 2 meters-sec⁻¹. FIG. 5shows the specimen of FIG. 4 after the application of the force. Testresults are reported as Crack Propagation Load in KiloNewtons (kN) andthe measured Energy in Joules (J) required to split apart the assembly.Test temperatures are reported as +23° C. and 90° C.

Test Method F: Sustained Load Durabilty (SLD)

This test is used to evaluate the relative durability of an adhesivebond when exposed simultaneously to a tensile load and environmentalaging. The test substrate is a 25 mm by 76 mm steel coupon (G60HD steelavailable from National Steel, unless otherwise indicated) that has beenlubricated with 61 MAL lubricant at a coating weight of 4.3 grams persquare meter (400 miligrams/ft²). The sample is prepared as for theoverlap shear test. The adhesive, containing 0.25 mm diameter glassbeads to control the bond line thickness, is applied to a 1.27 cm longarea on the oiled side of one coupon. A second oiled coupon is placedover the adhesive and the sample is clamped. The sample measures 14 cmin length with an overlap adhesive bond measuring 1.27 cm in the middle.Excess adhesive that squeezes out of the edges is removed prior tocuring. The sample is cured in an oven at 163° C. for 20 minutes. Thesample is then allowed to equilibrate at room temperature beforetesting.

Each end of the sample is punched with a 6.35 mm hole that is centered1.27 cm in from each end. For a single test, five samples are arrangedand bolted end to end in alternating fashion so that the bondlines arealigned along the center of the test fixture. Stainless steel bolts(6.35 mm dia. by 19.05 mm long) with corresponding nuts and nylonwashers having a 19.05 mm diameter were used to bolt the samplestogether to form a string of five samples.

The test fixture is a stainless steel U-channel measuring 63.5 cm longby 5.1 cm wide by 2.5 cm high. The walls of the U-channel are 0.3 cmthick. The U channel has a spring attached to one end and a fixed endblock attached to the other end. The fixed end block is a 5.7 cm by 4.3cm steel block having a 3 cm by 4.3 cm by 1 cm thick block cut out ofone end to form a step in the block. The stepped end block fits into thechannel with the stepped portion facing the inside length of thechannel. The end block is bolted to one end of the U-channel and a boltaffixed at the center of the stepped portion of the end block is used toattach the test samples. The other end of the U-channel has a fixed endcap with a 24 mm by 1.2 cm diameter threaded rod extending through it.Within the channel, the rod is attached to an end block similar to theone on the opposite end except that this end block is free to move asthe threaded rod is turned. A 304 coiled stainless steel spring havingan I.D. of about 2.5 cm, a length of 9.7 cm, and a spring rate ofapproximately 15 kg-mm⁻¹ (available from Century Spring Corp., LosAngeles, Calif.—part # is RV-43190), with fitted endcaps and washers oneach end of it, is placed over the threaded rod. A hex nut is placedover the threaded rod to hold the spring in place. A hollowed outcylinder having an inside diameter that is slightly larger than theouter spring diameter is placed over the spring to prevent lateraldeflection of the spring. The amount of deflection in the spring equalto 226.8 kilograms was determined by compressing the spring on anInstron 4210 compression tester to that weight and measuring thecompressed length of the spring. The compressed length was approximately85% of its original length.

For the test, the string of samples is bolted to each of the end blocksinside of the U-channel and the spring is compressed to the calibratedcompression length to yield a tensile stress of 7 MPa. The rack ofsamples under a tensile stress are aged in the test cycle below. One dayrepresents one cycle (i.e., 24 hours). The test is typically started ona Monday morning, and five cycles are run during the week (i.e., fivedays a week). The rack is left in the controlled environment cabinetover the weekend with no immersion in the salt solution. Weekend daysare not counted as cycles. Test results are reported in cycles. Thesteps of each cycle are as follows:

1. The rack is first immersed in a salt solution (5% by weight sodiumchloride in distilled or deionized water) at 23±2° C. for 15 minutes.

2. The rack is then removed from salt solution and allowed to drip dryvertically at 23±2° C. for 105 minutes.

3. The rack is next placed in a controlled environment cabinet at 50±2°C. and 90±5% relative humidity for 22 hours. The rack is checked dailyfor failure. If one of the lap shear samples in a rack fails, the sampleis removed and a spacer is bolted in its place to maintain theappropriate stress.

4. Cycling is continued until three bonds have failed, noting the cyclesto failure for each bond. The number of cycles to failure for the threebonds is averaged and then recorded.

Test Method G: Fatigue Test

This test is a measure of cyclic fatigue resistance as measured by thetotal number of cycles that an adhesive bond endures and the amount ofcrack propagation in the bond. Test samples are prepared as for TestMethod B (T-peel Adhesion) described above except that the coupons arebent 90 degrees to form a 2.5 cm tab where the bend radius is half ofthe metal thickness. The coupons are 0.7 mm thick 70G70GE draw qualityelectro-galvanized steel from National Steel. The bent coupons aredegreased as described above in steps 1), 2), 3) and 6) of the FPLEtched Aluminum substrate. Coupons are drip-dried for 10 minutes atambient temperature, then placed in a recirculating air oven at 54° C.for 30 minutes to dry. The dried coupons are placed in a sealedpolyethylene bag and stored in a dessicator until the bonds are made.

Prior to application of the adhesive, a 2.5 cm wide strip of 5419 KaptonTape available from 3M Company, St. Paul Minn., is applied across theend of the tab on the adhesive side of each coupon so that the tab isessentially covered by the tape. The adhesive is then applied, asdescribed above for Test Method B, to the untaped surface of the tapedside of each coupon up to the edge of the tape. The adhesive coatedsurfaces of two such coupons are brought together and clamped with twomedium sized binder clips along each edge of the coupons. Excessadhesive that squeezes out is removed. The clamped assembly is thenplaced in a forced air oven set at 163° C. for 20 minutes to cure theadhesive. The tape is not removed from the tabs. The test assembly isallowed to equilibrate at room temperature prior to testing.

The test is conducted on an MTS 880 tensile testing machine set in aconstant load mode for a 20 Hertz sinusoidal cycle with +222.4 Newtonsforce for maximum load and 22.2 Newtons for minimum load. The tabbed(i.e., taped) ends of the bond assembly are inserted into the grips andthe grip edges are positioned equidistant from the center of thebondline. The test assembly is preloaded to a 0.055 mm displacementprior to initiating cycling. Automatic termination of the cycling wouldoccur if vertical displacement exceeded 6.35 mm. The adhesive of Example172 exhibited 3,391,000 cycles at which time the test was manuallyterminated with negligible crack propagation (less than 1 mm).

Identification of Components Used in the Examples

Epon™ 828 Epoxy Resin—diglycidyl ether of Bisphenol A having an epoxyequivalent weight of about 190 and an average molecular weight of350-400, and available from Shell Chemical Company.

Epon™ 58006 resin (Toughener)—Epoxy resin adduct having 40% by weightHycar 1300X8 and 60% by weight Epon 828 available from Shell ChemicalCompany

Paraloid™BTA IIIF copolymer (Toughener)—methylmethacrylate/butadiene/styrene copolymer available from Rohm & HaasCompany.

PARALOID™ EXL2600 (Toughener)—Methacrylate/butadiene/styrene core-shellpolymer available from Rohm & Haas.

MK107 Reactive Diluent—diglycidyl ether of cyclohexane dimethanolavailable from Shell Chemical Company.

GP-71 silica—silicon dioxide having a particle size in the range of fromabout 20 to about 30 micrometers, available from Harbison-Walker Corp.

Cab-0-Sil™ TS-720 silica—fumed silica available from Cabot Corp.

“B37/2000” glass bubbles—glass bubbles available from Minnesota Mining &Manufacturing Company.

Glass Beads—solid glass beads having a diameter of 0.25 mm±0.01 mm,obtained from Cataphote, Inc., Jackson Miss.

Other chemical compounds used can be obtained from chemical supplycompanies such as Aldrich Chemicals.

EXAMPLES 1-20

A first epoxy resin premix composition (Premix I) was prepared by mixing500 grams of Epon™ 828 epoxy resin with 125 grams of Paraloid™EXL2600copolymer using a high shear mixer between 110-120° C. for about 30minutes, and then cooling to ambient temperature. A second epoxy resinpremix composition (Premix II) was prepared by combining 243 grams ofEpon™ 828 epoxy resin with 130 grams of catechol in a glass jar,flushing with nitrogen, and heating at 121° C. for 15 minutes withoccasional stirring until a clear homogeneous (i.e., no apparent phaseseparation or recrystallization) solution was formed. The mixture wascooled to ambient temperature. Part B of an epoxy resin composition wasprepared by mixing 335 grams of Premix I, 339 grams of Premix II, 65grams of MK107 reactive diluent, 201 grams of GP-71 silica, 30 grams ofK37 glass bubbles, 17 grams of Cab-0-Sil™ TS-720 silica, and 12 grams ofglass beads in a planetary mixer under vacuum for about 20 minutes. Theresulting composition had a smooth paste-like consistency.

Two-part epoxy adhesive compositions were prepared by mixing varyingamounts of Part A (catalyst only) shown in Table 1 and 5.0 grams of PartB. The amounts of catalyst are shown as a percent of the totalformulation (%T), by weight in grams (Part A—grams) and also as apercent of the reactive species (% Cat), i.e., the amounts of epoxy,catechol, and amine from the catalyst. The catalyst used for Examples1-7 was 2-ethyl-4-methylimidazoline; the catalyst for Examples 8-14 was2-benzyl-2-imidazoline; the catalyst for Examples 15-20 was4,4-dimethyl-2-imidazoline. The active hydrogen to epoxy molar ratio forthese examples (i.e., OH-Amine/Epoxy ratio) was maintained at about 0.8for each of these examples. In calculating this ratio, any aliphatichydroxyls present in an exemplary amine (e.g., 3-amino-1-propanol) werenot considered. The “OH” referred to in the OH-Amine/Epoxy ratio refersto a phenolic OH (i.e., from the phenolic chain extender), and the“Amine” refers to any NH₂ and/or NH from the amine chain extender andcatalyst.

The adhesives were tested for Impact Peel Resistance, as measured bycrack propagation load in kiloNewtons (kN) and total energy in Joules(J) at 23° C. and 90° C. according to the test described above. Theadhesives were also tested for T-peel adhesion at 23° C. on the abovedescribed steel substrate. Test results are shown in Table 1.

TABLE 1 Energy Load Energy Load T-Peel % Part A (J) (kN) (J) (kN) N-cm⁻¹Ex T grams % Cat @ 23° C. @ 23° C. @ 90° C. @ 90° C. @ 23° C. 1 1 0.051.47 NT NT NT NT 0 2 2 0.10 2.88 2 0.1 27 0.8 2 3 3 0.15 4.27 15 0.4 220.7 114 4 4 0.20 5.62 17 0.5 27 0.8 128 5 5 0.25 6.93 12 0.4 24 0.7 1146 6 0.30 8.20 2 0 17 0.5 2 7 7 0.35 9.44 0 0 0 0.1 0 8 1 0.05 1.47 0 0NT NT 0 9 2 0.10 2.89 0 0 NT NT 0 10 3 0.15 4.27 8 0.3 NT NT 114 11 40.20 5.62 5 0.2 NT NT 88 12 5 0.25 6.93 16 0.5 NT NT 105 13 6 0.30 8.2015 0.5 NT NT 105 14 7 0.35 9.44 11 0.3 NT NT 93 15 1 0.05 1.47 0 0 NT NT0 16 2 0.10 2.89 2 0.1 NT NT 67 17 3 0.15 4.27 9 0.3 NT NT 102 18 4 0.205.62 12 0.4 NT NT 119 19 5 0.25 6.93 2 0.1 NT NT 44 20 6 0.30 8.20 1 0.1NT NT NT NT = Not Tested

data in Table 1 show that an imidazoline catalyst can provide superiorImpact Peel Resistance at 23° C. and 90° C., as well as superior T-peeladhesion at 23° C., over an optimum concentration range of catalyst.

EXAMPLES 21-38

Part B of an epoxy adhesive composition was prepared as in Examples1-20. An epoxy adhesive was prepared by mixing 5.0 grams of Part B withvarying amounts and types of tertiary amine and cyclic amidine catalystsas shown in Table 2. The amounts of catalyst are shown in grams and as aweight percent of the total weight of the reactive species (epoxy,catechol, and catalyst) and ranged from 1.47% to 9.44%. TheOH-Amine/epoxy ratio was maintained at about 0.8. The adhesivecompositions were tested adhesion on steel substrates, as describedabove.

Examples C1-C5

The adhesives of Examples C1-C5 were prepared in the same manner asExamples 21-38 except that the catalysts used that did not result in asuitable adhesive. The specific compounds and corresponding test dataare shown in Table 2.

TABLE 2 Ex Catalyst T-Peel - N-cm⁻¹ at varying % Catalyst CatalystConcentration - grams .05 0.10 0.15 0.20 0.25 0.30 0.35 CatalystConcentration - wt % 1.47 2.88 4.27 5.62 6.93 8.20 9.44 212-Ethyl-4-methylimidazoline NT 2 114 128 114 2 0 222-Benzyl-2-imidazoline 0 0 114 88 105 105 93 234,4-Dimethyl-2-imidazoline 0 67 102 119 44 NT NT 241,4-Diazabicyclo<5.4.0>undec-7-ene 0 0 23 88 110 50 NT 251,5-Diazabicyclo<4.3.0>non-5-ene 0 47 79 88 84 NT NT 261,4-Diazabicyclo<2.2.2>octane 0 0 39 NT NT NT NT 27 1-Acetylimidazole123 117 43 26 NT NT NT 28 2-Ethyl-4-methylimidazole 70 67 70 67 NT NT NT29 1-Benzyl-2-methylimidazole 0 53 58 61 63 NT NT 30 1-Butylimidazole 6556 63 53 NT NT NT 31 1-Butylpyrrolidine 35 65 78 79 78 NT NT 321-(2-Aminoethyl)piperidine 0 44 44 53 NT NT NT 33 1-Vinylimidazole 44 5356 60 61 NT NT 34 1,4,5,6-Tetrahydropyrimidine 0 96 93 82 NT NT NT 351-Allylimidazole 40 61 61 63 65 NT NT 36 4-(4-Methylpiperidino)pyridine44 66 67 67 NT NT NT 37 1,2-Dimethylimidazole 35 63 53 44 NT NT NT 38N,N′-Dimethylbenzylamine 31 43 53 61 60 NT NT C1 Tributylamine 0 0 0 0NT NT NT C2 1-Phenylimidazole 0 0 0 0 NT NT NT C32-Ethyl-4-methylthiazole 0 0 0 0 NT NT NT C4 1-Methylindole 0 0 0 0 NTNT NT C5 2-Phenyl-2-imidazoline 0 0 0 0 0 0 0 NT = Not tested

The results in Table 2 show how the T-peel adhesion performance of theepoxy adhesive ns of the invention can vary by using different amountsand types of catalysts.

Examples 21, 24, 31, 34, and 38, at varying concentrations of catalystbased on the weight the reactive species (% Cat) shown in Table 3, werealso tested for Impact Resistance as measured by the crack propagationload and total energy at 23° C. and 90° C., and Overlap Shear Adhesionat 121° C. (Shear—MPa) on the steel substrate.

TABLE 3 Load Overlap Energy (kN) Energy Load Shear % (J) @ (J) (kN)(MPa) Ex Catalyst @ 23° C. 23° C. @ 90° C. @ 90° C. @ 121° C. 21 1.47 00 9 0.4 1.1 21 2.89 6.7 0.2 20 0.7 4.8 21 4.27 13 0.3 22 0.6 2.9 21 5.6212 0.4 24 0.8 4.3 21 6.93 11 0.3 20 0.6 6.2 21 8.20 2 0.1 18 0.6 4.8 219.44 0 0 16 0.5 4.7 24 2.88 0 0 14 0 1.1 24 4.27 1 0 NT NT 3.5 24 5.6211 0.3 NT NT 2.9 24 6.93 9 0.3 14 0.4 3.3 24 8.20 4 0.2 14 0.5 3.9 249.44 2 0.1 14 0.5 5.3 31 2.88 8 0.2 10 0.3 2.9 31 4.27 8 0.3 14 0.5 2.231 5.62 9 0.3 14 0.5 2.4 31 6.93 9 0.3 15 0.5 2.0 34 2.88 11 0.3 20 0.64.2 34 4.27 13 0.4 16 0.5 4.8 34 5.62 4 0.1 14 0.4 5.0 34 6.93 2 0 110.3 5.6 34 8.20 2 0.1 13 0.4 6.2 34 2.88 7 0.3 9 0.3 1.8 38 4.27 7 0.311 0.3 2.2 38 5.62 9 0.3 13 0.5 1.9 38 6.93 7 0.3 13 0.4 2.0 38 8.20 80.3 14 0.5 1.6 NT = Not tested

Table 3 show the impact peel resistance, at room and elevatedtemperature, and the overlap shear strength, at elevated temperature, ofthe compositions of the invention can vary by using different amountsand types of the catalysts of the present invention.

The catalysts of Examples 21-38 fall within the broad classes ofcatalysts described as substituted cyclic amidines (Exs. 21-25, 27-30,33-35 and 37), tertiary amines (Exs. 26 and 38). pyrrolidines (Ex. 31),piperidines (Ex. 32), or pyridines (Ex. 36). Some cyclic amidines aremore sensitive to substitution chemistries than others. Somesubstituents can help and other substituents can hurt the catalystperformance of a cyclic amidine. The effectiveness of a cyclic amidinecan be severly impaired by the wrong substitution chemistry. Inaddition, what substituent is on the nitrogen or adjacent to thenitrogen of the cyclic amidine linkage can determine the degree ofcatalytic activity exhibited by the catalyst. It is very difficult topredict the effect of a particular substitution chemistry and is,typically, determined by trial and error experimentation. While cyclicamidines can be particularly sensitive to the electron withdrawingeffect of a substituent, they may also be sensitive to stereo chemicaleffects, such as steric hinderance. Tertiary amines can be particularysensitive to a substituent that exhibits a high degree of sterichinderance; therefore, dimethyl substitution can be preferred. Ingeneral, pyrrolidines, piperidines and pyridines begin as less effectivecatalysts, and could become weaker when substituted.

The catalysts of Examples C1-C5 either were too sterically hindered(C1), contained too strong of an electron withdrawing group (C2 and C5),or were otherwise ineffective as a catalyst (C3 and C4) because of theirinherent chemistry. In addition to containing a strong electronwithdrawing group, the catalyst of Example C2 may also be stericallyhindered and/or may not have the proper solubility. Some of the othercatalysts used in the Table 2 examples may also be unacceptable forcertain applications. For example, the adhesive of Example 26 may not besuitable (e.g., strong enough) for some structural bonding applications.In addition, the adhesives of Examples 28-33 and 35-38 may not besuitable (e.g., strong enough) for other structural bondingapplications.

Another way that a catalyst can be ineffective is if it is insoluble ornot adequately soluble in the Part A and B blend. A catalyst can also beineffective or less effective when used to bond some substrates, butvery effective when used to bond other substrates. This difference ineffectiveness can be caused, at least in part, by the adhesive curingtoo quickly and not allowing sufficient time for the adhesive tosufficiently wet to the substrate surface. If it cures too quickly, theadhesive may not have the time needed to adequately wet out and bond toa particular substrate, thereby reducing the overall bond strength. Somesubstrates are less affected by rapid cure times, compared to othersubstrates. For instance, the etched aluminum substrates described abovecan be less sensitive to cure times than the galvanized steel substratesdescribed above. In particular, even though Examples 46 and 49 use thesame catalysts as that found in Examples 38 and 36, respectively, theadhesives of Examples 46 and 49 (i.e., bonded to the etched aluminum)exhibit dramatically improved T-peel adhesion compared to the adhesivesof Examples 38 and 36 (i.e., bonded to the galvanized steel). There isalso a significant improvement in the T-peel adhesion exhibited by theadhesive of Example 50 (i.e., bonded to the etched aluminum) compared tothat of Example 39 (i.e., bonded to the galvanized steel), even thoughthey are both imidazole catalyzed adhesives.

EXAMPLES 39-42

A Part B premix composition was made by adding 191 grams of Epon 828, 70grams of MK107, 307 grams of the earlier Part B premix compositiondescribed for Examples 1-20, 154 grams of Shell resin 58006 and 122grams of catechol to a glass jar, flushing with N2 then placing in anoven at 121° C. These components were allowed to melt with occasionalagitation to form a homogeneous solution. This was then allowed to coolto ambient temperature, and 767 grams were transferred to a planetarymixing bowl. To this was added 196 grams of GP7I, 16.9 grams of TS720,6.8 grams of K37 and 13.5 grams of glass beads. This was then mixedunder vacumn for 20 minutes to a smooth, paste like consistency. To 5gram portions of the resulting Part B composition were added respectiveamounts of each catalyst, as specified in Table 4. The amount of eachcatalyst is indicated in grams and % by weight of the reactive species.The adhesive compositions were tested for T-peel adhesion on steelsubstrates, as described above.

TABLE 4 Ex Catalyst T-Peel - N-cm⁻¹ at varying % Catalyst CatalystConcentration - grams 0.05 0.1 0.15 0.20 0.30 0.40 CatalystConcentration - wt % 1.51 3.02 4.51 6.01 7.51 9.01 39 Imidazole 114 7935 NT NT NT 40 1-Phenylimidazole 5 NT 10 NT 26 NT 41 2-Phenylimidazole93 105 88 NT NT NT 42 1,2-Dimethyl-1,4,5,6-Tetrahydropyrimidine NT 88 NT140 NT 18 NT = Not tested

The catalyst of Example 42 (i.e.,1,2-dimethyl-1,4,5,6-tetrahydropyrimidine) is manufactured by KoeiChemical Company of Osaka, Japan. As can be seen from the data in Table4, the location of the phenyl substitutent on the imidazole ring canhave a significant effect. In the 1 position, the electron withdrawingpower of the phenyl substitutent is powerful enough to render thecatalyst ineffective, as evidenced by the low T-peel strength (see alsoExample C2). In the 2 position, the electron withdrawing power of thephenyl substituent is much less, as evidenced by the relatively highT-peel strength. The electron withdrawing effect of the phenyl group ismore significant in the 2 position on the imidazoline ring (i.e.,substitution in the 2 position has more of a detrimental affect withimidazoline than with imidizole), as shown by Example C5. The type andlocation of a substituent can have more or less of an affect on theproperties of the adhesive.

EXAMPLES 43-51

An epoxy resin premix composition was made by mixing 1016 grams of Epon828 and 194 grams of Paraloid BTA IIIF core shell copolymer at an 84/16weight ratio in a moderate shear mixer at 110° C. to 120° C. for about 1hour. The mixture was substantially free of gel particles. Part B of anepoxy adhesive composition was prepared by placing 1000 grams of thepremix into ajar with 190 grams of catechol. The jar was flushed withnitrogen and placed in an oven at 121° C. for about 30 minutes, withoccasional agitation until the catechol disolves. The mixture was thencooled. Epoxy adhesive compositions were prepared by mixing portions ofPart B with various catalysts in amounts over the range ofconcentrations shown in the Table 5. The amounts of catalyst were variedfrom 0.5% to 6.4% as shown in Table 5 (Catalyst Concentration wt-%). Thecatalyst concentration is the percent of catalyst based on the totalweight of the reactive species, i.e., the amounts of epoxy, catechol,and catalyst at an OH/Epoxy ratio of about 0.8. The adhesives weretested for T-peel adhesion to FPL etched aluminum substrates using thetest method described above.

TABLE 5 Ex Catalyst T-Peel Adhesion (N-cm⁻¹) Catalyst Concentration -grams 0.5 1.0 2.0 3.0 4.0 5.0 6.0 7.0 Catalyst Concentration - wt % 0.480.96 1.90 2.82 3.74 4.63 5.50 6.36 43 2-Ethyl-4-methylimidazole NT 5 165175 NT 2 NT NT 44 2-Ethyl-4-methylimidazoline NT 2 166 179 184 210 5 445 1,4-Diazabicyclo<5.4.0>undec-7-ene NT 2 2 2 2 175 168 7 46N,N′-Dimethylbenzylamine NT NT 172 172 168 173 172 NT 47Dimethylethanolamine NT NT 175 170 11 2 2 NT 48Bis(2-dimethylaminoethyl)ether NT NT 172 158 147 9 11 NT 494-(4-Methylpiperidino)pyridine NT NT 14 151 156 161 NT NT 50 ImidazoleNT 140 158 9 2 NT NT NT 51 4-Dimethylaminopyridine 2 9 131 128 79 81 NTNT

The data in Table 5 show how the T-peel adhesion to FPL etched aluminumof epoxy adhesive compositions of the invention can vary by usingdifferent amounts and types of catalysts. TheBis(2-dimethylaminoethyl)ether catalyst is a commercially availablecatalyst manufactured by OSI Specialties Incorporated, of Danbury,Conn., under the name Niax A99. The T-peel adhesion of the Example 48adhesive can be much less when bonded to a galvanized steel substrate,like that described above (i.e., less than half that shown in Table 5for about the same catalyst concentrations).

EXAMPLES 52-58

An epoxy resin premix composition was prepared by mixing 500 grams ofEpon™ 828 epoxy resin with 125 grams of Paraloid™EXL2600 copolymer usinga high shear mixer at about 110° C. for about 30 minutes, and thencooling to ambient temperature. Part B of an epoxy adhesive was preparedby mixing 380 grams of the premix composition, 251 grams of Epon™ 828epoxy resin, 73 grams of MK107 reactive diluent, 229 grams of GP-71silica, 34 grams of K37 glass bubbles, 20 grams of Cab-0-Sil™ TS-720silica, and 14 grams of glass beads in a planetary mixer under vacuumfor about 20 minutes.

A catalyst composition (Part A) was prepared by adding 60 grams ofcatechol and varying amounts of 2-ethyl-4-methylimidazoline as thecatalyst (Catalyst Amt—gms) shown in Table 6 to ajar, flushing withnitrogen, and capping the jar. The resulting catalyst composition washeated in an oven at 125° C. with occasional agitation to form a Part A.The Part A was then cooled to ambient temperature. An epoxy adhesivecomposition was prepared by mixing 5.0 grams of Part B with varyingamounts of Part A shown in Table 6. The specific amount of catalyst ingrams (Catalyst Amt—gms) is shown in Table 6 as well as the specificamount of Part A in grams, and the amount of catalyst as a weightpercent of the reactive species, i.e., epoxy, catechol, amine, andcatalyst (% Cat). The OH-Amine/Epoxy ratio was maintained at about 0.8.The adhesives were tested for Impact Peel Resistance at 23° C. and 90°C. and Overlap Shear Adhesion at 121° C. to the galvanized steelsubstrates, as previously described, and the test results are shown inTable 6.

TABLE 6 Energy Load Energy Load Overlap Shear Catalyst Part A (J) (kN)(J) (kN) (MPa) Ex Amt - gms (gms) % Cat. @ 23° C. @ 23° C. @ 90° C. @90° C. @ 121° C. 52* 4.7 0.81 1.46 0 0 NT NT 0.6 53* 9.9 0.84 2.89 7 NRNT NT 4.5 54 15.2 0.88 4.33 24 0.8 29 0.9 5.6** 55 21.4 0.91 5.75 24 0.732 1.0 6.6** 56 28.1 0.94 7.16 21 0.6 32 1.1 7.0** 57 35.6 0.97 8.59 70.3 28 0.8 5.4** 58 43.9 1.00 10.00 5 0.1 24 0.8 NT *The solution ofPart A was kept warm at around 75° C., after removal from the oven, andthe Part B was warmed to @ 75° C., before mixing the two parts, in orderto prevent the Catechol from recrystallizing. **Results are from adifferent set of samples then that used for the impact data. NT - nottested NR - not recorded

The data in Table 6 show how Impact Peel Resistance and Overlap ShearAdhesion can vary using a mixture of different amounts of a preferredcatalyst and a constant amount of catechol. It is undesirable for thecatechol to recrystallize. Table 6 also shows that for a2-ethyl-4-methylimidazoline catalyst, the catechol is less likely torecrystalize at higher concentrations of the catalyst.

EXAMPLES 59-68

A catalyst composition (Part A) was prepared by adding 60 grams ofcatechol, 40 grams of 3-amino-1-propanol and varying amounts ofimidazole in grams (Cat—gms) as shown in Table 7 to ajar and heating inan oven at 121° C. with stirring for about 10 minutes. The Part A wasthen cooled to ambient temperature. Two part epoxy adhesives wereprepared by mixing about 5 grams of Part B as is described in Examples52-58 with varying amounts of Part A in grams (Part A—gms) shown inTable 7. The amounts of catalyst also shown as a percent of the reactivespecies (% Cat), i.e., epoxy, catechol, catalyst. The OH-Amine/Epoxyratio was maintained at about 0.75 for Examples 59-62 and at 0.8 forExamples 63-68.

The adhesives were tested for Impact Peel Resistance, T-Peel Adhesion at23° C., and Overlap Shear Adhesion at 121° C. on steel as describedabove. Test results are shown in Table 7. Comparative Examples C6-C8 arestate of the art epoxy adhesives that are used commercially in theautomotive industry and test results are shown in Table 7. ComparativeExample C6 is a structural one-part epoxy adhesive manufactured forChrysler Corporation under the name MSCD 457B by Cemedine, U.S.A. Inc.of Oak Creek, Wis., C7 is a structural one-part epoxy adhesivemanufactured for Chrysler Corporation under the name MSCD 457C by PPGIndustries of Adrian, Mich., and C8 is a structural one-part epoxystructural adhesive manufactured for General Motors under the name998-1989 by PPG.

TABLE 7 Overlap Shear T-Peel Energy Ex. Cat. Gms Part A gms % Cat. MPa @121° C. N-cm⁻¹ @ 23° C. J @ 23° C. 59 4.8 0.63 0.75 NT 88 16 60 9.9 0.641.49 NT 70 15 61 21.3 0.66 2.98 NT 96 4 62 34.5 0.68 4.47 NT 91 4 63 2.20.66 0.37 3.6 26 1 64 4.5 0.67 0.75 4.3 91 18 65 6.9 0.67 1.13 3.8 93 1766 9.3 0.68 1.50 3.6 79 20 67 11.9 0.68 1.88 4.1 105 20 68 14.5 0.682.25 4.8 102 16 C6 NA NA NA 1.6 32 2 C7 NA NA NA 2.5 35 2 C8 NA NA NA8.9 26 4 NT - Not Tested NA - Data not applicable

The data in Table 7 show that adhesives of the invention can have anamount of imidazole as a catalyst which provide good Impact PeelResistance and that formulations can be made which are superior overstate of the art structural adhesives. Table 7 also shows that theImpact Peel Resistance of the adhesive can be more sensitive to (i.e.,more adversely impacted by) increases in catalyst concentration thanT-peel strength. So, the T-peel strength can be acceptable while theImpact Peel Resistance is not.

EXAMPLES 69-72

Part A of an epoxy adhesive composition was prepared as in Examples63-68 except with varying amounts (in grams) of imidazole shown in Table8. The other components of Part A were 60 grams of catechol and 40 gramsof 3-amino-1-propanol as described above.

Part B of an epoxy adhesive composition was prepared as in Examples52-58. Epoxy adhesives were prepared by mixing about 5 grams of Part Bwith each Part A, containing a different amount of imidazole, shown inTable 8. The amounts of imidazole are shown in grams, as a percent ofthe reactive species (% Cat) and as a percent of the epoxy containingspecies (% Cat/Epoxy), i.e., Epon™ 828 and MK107. The OH-amine/Epoxyratio was about 0.8. The adhesives were tested for T-Peel Adhesion onsteel substrates at 23° C., as described above.

Comparative Example C9

An epoxy adhesive was prepared as in Example 69-72 except that theamount of imidazole was 0.25% of the epoxy materials, (% Cat/Epoxy) or0.21% of the reactive species (% Cat). The adhesive was tested as inExamples 69-72.

TABLE 8 Imidazole Part A % T-Peel Ex grams grams % Cat Cat/Epoxy N-cm⁻¹C9 1.3 0.62 0.21 0.25 0 69 1.9 0.62 0.36 0.30 5 70 2.8 0.62 0.53 0.44 7971 3.3 0.62 0.62 0.52 74 72 4.8 0.63 0.89 0.75 79

The data in Table 8 indicates that useful amounts of imidazole will beabove about 0.35% and, preferably, above about 0.5%, based on thereactive species.

EXAMPLES 73-81

Two-part adhesives were prepared by mixing about 5 grams of Part Bdescribed in Examples 52-58 with varying amounts and compositions ofPart A (Part A—grams) shown in Table 9. The OH-Amine/Epoxy ratio waskept constant at about 0.8. The amounts of catechol (Catechol grams),3-amino-1-propanol (Amine grams) and 2-ethyl-4-methyl imidazoline(Catalyst grams) were varied as shown in Table 9. The amounts ofcatalyst (i.e., 2-ethyl-4-methylimidazoline) as a percent by weight ofthe reactive materials (% Cat) in the formulations is also shown.

Part A was prepared by mixing the catechol, amine, and catalyst (i.e.,2-ethyl-4-methyl imidazoline) in ajar, flushed with nitrogen, andheating in an oven at 121° C. with occasional agitation for about 10minutes. The Part A was then cooled to ambient temperature before mixingwith Part B.

The adhesives were tested for T-peel adhesion at 23° C. on cleaned andlubricated (i.e., lubed) steel, as described above.

TABLE 9 T-peel Adhesion Catechol Amine Catalyst Part A N-cm⁻¹ Ex gramsgrams grams Grams % Cat Cleaned Lubed 73 80 20 22.9 0.79 3.70 137 137 7480 20 33.6 0.82 5.14 137 122 75 80 20 45.4 0.86 6.59 130 131 76 60 4024.6 0.74 3.70 131 105 77 60 40 36.0 0.78 5.15 140 136 78 60 40 48.80.81 6.62 145 140 79 40 60 26.3 0.70 3.71 131 105 80 40 60 38.5 0.745.17 158 114 81 40 60 52.2 0.77 6.64 158 119

The data in Table 9 show that at a preferred stoichiometric ratio ofOH-Amine/Epoxy of about 0.8, the adhesives of the invention can exhibitsuperior T-peel adhesion on both clean and lubricated steel over therange of the catechol to amine weight ratio and catalyst percentage.

EXAMPLES 82-93

These examples were prepared as in Examples 73-81 except that thestoichiometric ratios (OH-Amine-Epoxy ratio) were varied from about 0.5to about 1.0, and the catechol and amine amounts were variedaccordingly. Part A was mixed with about 5 grams of Part B described inExamples 52-58. The amounts and compositions of Part A are shown inTable 10. The catalyst (2-ethyl-4-methylimidazoline) level was adjustedand is shown as a percent of the reactive species (% Cat). The adhesiveswere tested for T-peel adhesion on both cleaned and lubricated (lubed)steel, as described above.

TABLE 10 T-peel Adhesion OH-Amine/ Catechol Amine Catalyst Part A N-cm⁻¹Ex Epoxy ratio grams Grams Grams Grams % Cat Cleaned Lubed 82 0.5 80 2055.5 0.56 5.27 40 79 83 0.6 80 20 45.4 0.65 5.22 56 96 84 0.7 80 20 38.50.74 5.18 119 119 85 0.8 80 20 33.6 0.82 5.14 136 123 86 0.9 80 20 29.90.92 5.11 49 61 87 1.0 80 20 27.0 1.0 5.08 0 0 88 0.5 60 40 60.0 0.535.27 44 79 89 0.6 60 40 48.8 0.61 5.22 53 79 90 0.7 60 40 41.5 0.69 5.2088 114 91 0.8 60 40 36.0 0.78 5.15 140 137 92 0.9 60 40 32.0 0.86 5.12157 131 93 1.0 60 40 29.0 0.95 5.11 0 0

The data in Table 10 show that the OH-Amine/Epoxy ratio can affect theT-peel results, independent of the catalyst concentration. In addition,at a relatively constant catalyst level, T-peel adhesion can be affectedby the test substrate, the amounts of catechol and amine, and theOH-Amine/Epoxy ratio.

EXAMPLES 94-132

For each of examples 94-132, a Part A catalyst composition was preparedby mixing 60 grams of catechol with the types and amounts of amines(Amine—grams) and the amounts of 2-ethyl-4-methylimidazoline(Catalyst—grams) shown in Table 11 in a jar. The jars were flushed withnitrogen, capped, and then placed in an oven at 121° C. for 10 minuteswith occasional agitation to form a homogeneous mixture. Thecompositions were then cooled. Part A for each of the examples has a 1:1molar ratio of catechol to amine. The amount of Part A shown in Table 11(Part A—grams) was mixed with about 5 grams of Part B described inExamples 52-58. The OH-Amine/Epoxy ratio was maintained at about 0.8 andthe amount of catalyst based on the reactive materials (% Cat) is shown.T-peel adhesion test results on cleaned steel, obtained according to thepreviously described test method, for all of the Examples 94-132 areshown in Table 11.

The adhesives of Examples C10-C15 were prepared as for Examples 94-132except that either the Part A recrystallized or the amine was insolublein the Part A. As a result, none of these Examples could be tested.

TABLE 11 Catalyst Part A % T-Peel Ex Amine Amine Grams Grams Grams CatN-cm⁻¹  94 3-Amino-1-propanol 41.0 34.0 0.77 4.76 140  952-Amino-2-methyl-1-propanol 48.5 34.5 0.81 4.78 105  96 Benzylamine 58.534.8 0.87 4.74 161  97 2-Methylbutylamine 47.4 34.3 0.81 4.77 152  98Isoamylamine 47.4 34.3 0.81 4.77 131  99 2-Amino-1-methoxypropane 48.634.3 0.81 4.75 131 100 2-(2-Aminoethoxy)ethanol 57.2 34.7 0.86 4.75 145101 Sec-Butylamine 40.0 34.0 0.76 4.75 131 102 Octylamine 71.0 35.0 0.944.68 145 103 Tridecylamine 109.0 36.8 1.16 4.65 119 1043-(Hexyloxy)-1-propylamine 91.5 36.0 1.06 4.82 123 105 Hexylamine 55.528.2 0.84 3.96 140 106 3-(di-n- 101.5 36.5 1.12 4.67 137Butylamino)propylamine 107 N,N′- 131.0 37.5 1.29 4.61 137Dibenzylethylenediamine 108 1-Methylbutylamine 47.4 34.3 0.81 4.77 140109 2-Ethylhexylamine 70.5 35.2 0.94 4.71 149 110 Isobutylamine 40.034.0 0.76 4.77 137 111 Ethanolamine 33.2 33.8 0.73 4.80 123 1126-Aminocapronitrile 61 34.8 0.89 4.73 161 113 4-Aminobenzylamine 66.635.1 0.92 4.73 128 114 Cyclohexylamine 54.5 34.5 0.85 4.72 126 115Oleylamine 151.5 38.3 1.4 4.52 128 116 Decylamine 86.5 36.0 1.0 4.69 131117 Dodecylamine 101.5 36.5 1.12 4.66 137 118 3-(1-Methylethoxy)-1- 67.635.0 0.92 4.71 163 propylamine 119 3-(Isodecyloxy)-1- 123.0 37.3 1.244.63 123 propylamine 120 4-(3-Aminopropyl)morpholine 78.5 35.0 0.99 4.65145 121 4-Amino-1-butanol 48.6 34.4 0.82 4.77 88 1221,8-Diamino-p-menthane 47.0 34.3 0.80 4.74 82 124 Aminomethylbutyne 46.334.3 0.80 4.77 79 125 Tris(hydroxymethyl)amino 66.5 35.0 0.92 4.72 70Methane 126 H221 (Dixie Chemical) 56.7 34.5 0.86 4.72 67 127 2-(2- 38.134.0 0.75 4.77 60 Aminoethylamino)ethanol 128 n-Butylamine 40.0 34.00.76 4.77 61 129 Aminodiphenylmethane 100.0 36.4 1.11 4.67 44 1301,10-Diaminododecane 47.0 25.8 0.78 3.71 44 131 Diethylenetriamine 22.933.4 0.67 4.78 35 132 3,3′-Diaminodipropylamine 28.4 33.5 0.70 4.79 26C10 Octadecylamine 147.0 38.0 1.38 4.57 R C11 6-Aminocaproic acid 71.535.2 0.95 4.71 I C12 4-Aminobutyric acid 56.2 34.7 0.86 4.75 I C1312-Aminodecanoic acid 117.5 37.0 1.21 4.63 R C14 t-Octylamine 71.0 35.30.94 4.71 R C15 Piperazine 46.9 25.8 0.78 4.66 I R = Part Arecrystallized - not tested I = Amine was Insoluble in the Part A - nottested

The data in Table 11 show that the choice of amine chain extender canimpact the performance (e.g., T-Peel Adhesion) of the resultingadhesive. Table 11 also shows that amines, which are useful as chainextenders in the practice of the invention, can include mono-primaryamines and secondary diamines that are not too sterically hindered onthe carbon alpha to the amine or on the amine itself and do not havestrong electron withdrawing groups on amine sites.

EXAMPLES 133-139

An epoxy resin premix composition was prepared by mixing 500 grams ofEpon™ 828 epoxy resin with 125 grams of Paraloid EXL2600 copolymer usinga high shear mixer at a temperature between 110° C. to 120° C. for about30 minutes and then cooling to ambient temperature. Part B was formed bymixing 330 grams of the epoxy premix, 164 grams of EPON™ 58006 resin,209 grams of Epon™ 828 epoxy resin, 76 grams of MK107 reactive diluent,231 grams of GP-71 silica, 8 grams of K37 glass bubbles, 20 grams ofCab-0-Sil™ TS-720 silica, and 15 grams of glass beads in a planetarymixer under vacuum for 20 minutes.

A Part A catalyst composition was prepared by mixing in jars 60 grams ofcatechol with the types and amounts of amines and2-ethyl-4-methylimidazoline catalyst (grams) shown in Table 12. Thenumber of reactive equivalents of amine was maintained relativelyconstant for all of the Examples 133-139 at 0.22. The jars were flushedwith nitrogen then placed in an oven at 121° C. for 10 minutes withoccasional agitation. The compositions were cooled to ambienttemperature. The amount of Part A shown in Table 12 (Part A—grams) wasmixed with 5.0 grams of the Part B described above. The OH-Amine/Epoxyratio was maintained at about 0.8 and the amount of catalyst (% Cat.) isshown based on the weight of the reactive species. The results of T-peeladhesion tests at 23° C. on steel, as described above, are also shown inTable 12.

TABLE 12 Amine Catalyst Part A % T-Peel Ex. Amine grams grams grams CatN-cm⁻¹ 133 3-Methoxy- 10.0 26.8 0.84 5.91 149 propylamine 134 2-Amino-1-10.0 26.8 0.84 5.91 140 methoxypropane 135 2-(2-Amino- 11.5 26.8 0.865.91 152 ethoxy)ethanol 136 3-(2-Methoxy- 14.5 27.0 0.89 5.91 152ethoxy)propylamine 137 3-Isopropoxypropyl 13.7 27.0 0.88 5.91 163 amine138 3-Isohexoxypropyl 18.5 27.2 0.92 5.88 158 amine 1393-Isodecoxypropyl 25.0 27.5 0.98 5.85 158 amine

The data in Table 12 show how the inventive adhesive can be formulatedso as to maintain OH-Amine/Epoxy ratio of about 0.8, while the molecularweight of the amine is increased, at a constant catalyst level. The dataalso show the utility of using ether amines with catechol as the chainextender.

EXAMPLES 140-153

A Part A catalyst composition was prepared by mixing in jars 80 grams ofcatechol and 20 grams of various amines with varying amounts of2-ethyl-4-methylimidazoline catalyst (Cat—gms) shown in Table 13. Thejars were flushed with nitrogen, placed in an oven a 121° C. for 10minutes with occasional agitation to form a homogeneous mixture, andthen cooled to ambient temperature. The amount of Part A in grams shownin Table 13 (Part A—gms) was mixed with 5.0 grams of the Part Bdescribed in Examples 133-139. The OH-Amine/Epoxy ratio was maintainedat about 0.8 and the amount of catalyst (% Cat.) is shown based onweight of the reactive species. T-peel adhesion and Impact PeelResistance test results measured as Load in kiloNewtons and Energy inJoules are shown in Table 13. All tests were performed on cleaned steelat 23° C., as described above.

TABLE 13 Part A % T-Peel Load Energy Ex Amine Cat gms gms Cat N-cm⁻¹ kNJ 140 3-Methoxypropylamine 39.0 0.83 5.95 149 0.6 18 1412-Amino-1-methoxypropane 39.0 0.83 5.95 145 0.5 16 142 2-Amino-1-butanol39.0 0.83 5.95 140 0.5 15 143 2-Amino-2-methyl-1 -propanol 39.0 0.835.95 126 0.4 14 144 3-Amino-1-propanol 41.0 0.80 6.02 145 0.6 18 145N,N′-Cyanoethylethylenediamine 35.5 0.90 5.95 131 0.6 18 146sec-Butylamine 41.0 0.80 6.00 135 0.5 15 147 2-Amino-1-methoxypropane39.0 0.84 6.00 131 NT NT 148 3-Ethoxypropylamine 38.0 0.86 6.01 137 NTNT 149 3-Isopropoxypropylamine 37.0 0.88 5.99 119 NT NT 1502-Ethoxyethylamine 39.0 0.83 5.99 140 NT NT 151 2-(2-Aminoethoxy)ethanol38.0 0.86 6.03 128 NT NT 152 1-Amino-2-propanol 40.5 0.81 6.03 131 NT NT153 3-(2-Methoxyethoxy)-propylamine 36.5 0.89 6.01 128 NT NT NT = Nottested

The data in Table 13 show additional useful amines in the practice ofthe invention. Examples 145 and 149 are amines manufactured by TomahProducts Inc. of Tomah, Wis. under the product designations Tomah 159-6and Tomah PA-7, respectively.

EXAMPLES 154-167

The exemplary epoxy adhesives were prepared by mixing 0.2 grams of2-ethyl-4-methylimidazoline with about 5 grams of the Part B of Examples52-58 plus the phenolic compounds of Table 14 in the amounts indicated.An OH-Amine/Epoxy ratio of about 0.7 was maintained. The compositionswere tested for T-Peel adhesion on steel at 23° C. and results are shownin Table 13. Comparative Examples C16-C21 were prepared as for Examples154-167 except using the phenolic compounds shown in Table 14.

TABLE 14 Amount - T-Peel - Ex Phenolic Grams N-cm⁻¹ 154 Catechol 0.68136 155 3-Fluorocatechol 0.8 93 156 3-Methylcatechol 0.78 117 1574-Methylcatechol 0.78 88 158 Resorcinol 0.69 53 159 3-Methoxycatechol0.88 117 160 1/1 equivalents catechol/resorcinol 0.34/0.34 137 161 1/1equivalents catechol/Bisphenol A 0.34/0.71 137 162 1/1.8 equivalentscatechol/Bisphenol A 0.23/0.91 131 163 2,3-Dihydroxynaphthalene 1.0 79164 None 0 26 165 3,5-Di-t-butylcatechol 1.39 46 166 Pyrogallic acid0.52 35 167 Octylpyrogallol 0.99 26 C16 2,3-Dihydroxybenzoic acid 0.64 *C17 3,4-Dihydroxybenzoic acid 0.64 * C18 3,4-Dihydroxybenzaldehyde0.86 * C19 4-Nitrocatechol 0.98 * C20 Gallic acid 0.54 0 C21 Laurylgallate 1.41 0 *Phenolic did not dissolve to make a homogeneous solution

the data in Table 14 show the affect of various phenolic chainextenders, including the use of no chain extender, on T-peel adhesionperformance.

EXAMPLES 168-194

Part B compositions for these two-part epoxy adhesives were prepared asfollows:

Composition I

Composition I was prepared by mixing 620 grams of Epon™ 828 epoxy resin,82 grams of MK 107 reactive diluent, 251 grams of GP-71 silica, 9 gramsof K37 glass bubbles, 22 grams of Cab-0-Sil™ TS-720 silica, and 16 gramsof glass beads in a planetary mixer for 20 minutes. The epoxideequivalent weight was 259.

Composition II

An epoxy resin premix composition was prepared by mixing 144 grams of apolytetramethylene oxide diamine (Toughener A), such as that disclosedin U.S. Pat. No. 3,436,359, issued Apr. 1, 1969 and incorporated hereinby reference, in 5 gram increments over a period of 10 minutes to 626grams of Epon™ 828 epoxy resin using a Meyers type mixing blade in aquart can at 100° C. Composition II was prepared by mixing 700 grams ofthe premix with 76 grams of MK 107 reactive diluent, 231 grams of GP-71silica, 8 grams of K37 glass bubbles, 20 grams of Cab-0-Sil™ TS-720silica, and 15 grams of glass beads in a planetary mixer for 20 minutesunder vacuum. The epoxide equivalent weight was 295.

Composition III

Composition III was prepared by mixing 327 grams of EPON™ 58006 resin(Toughener B), 372 grams of Epon™ 828 epoxy resin, 76 grams of MK 107reactive diluent, 231 grams of GP-71 silica, 8 grams of K37 glassbubbles, 20 grams of Cab-0-Sil™ TS-720 silica, and 15 grams of glassbeads in a planetary mixer under vacuum for about 20 minutes. Theepoxide equivalent weight was 302.

Composition IV

An epoxy resin premix composition was prepared by mixing 140 grams ofParaloid EXL2600 copolymer (Toughener C) and 560 grams of Epon™ 828epoxy resin as described above for Examples 52-58. Composition IV wasprepared by mixing 655 grams of the premix composition, 45 grams ofEpon™ 828 epoxy resin, and 75 grams of MK 107 reactive diluent, 231grams of GP-71 silica, 8 grams of K37 glass bubbles, 20 grams ofCab-0-Sil™ TS-720 silica, and 15 grams of glass beads in a planetarymixer under vacuum for about 20 minutes. The epoxide equivalent weightwas 295.

A premix for a Part A catalyst composition was prepared by combining 228grams of 2-ethyl-4-methylimidazoline, 5 grams of a poly tetramethyleneoxide diamine, and 625 grams catechol in ajar, flushed with nitrogen,and then capped. 818 grams of the resulting premix composition washeated in an oven at 121° C. with occasional vigorous agitation over aperiod of 30 minutes to form a homogenous solution. After cooling toambient temperature, the premix was transferred to a planetary mixerbowl and 12 grams of carbon black (available from DeGussa PigmentsDivision of Teterboro, N.J., under the product name Printex 3), 14 gramsof GP7I silica, 21 grams of TS720 silica and 4 grams of K37 glassbubbles were added. The Part A composition was mixed under vacuum for 20minutes. Because the amount of catechol is about three times the amountof catalyst, there may be a risk of this Part A compositionrecrystallizing. To avoid this risk and still maintain a high content ofcatechol, the amount of catechol in the Part A can be reduced andcatechol added to the Part B.

Epoxy adhesive compositions for Examples 168-194 were prepared by mixing1.0 gram of Part A with varying amounts of Part B Compositions in gramsshown in Table 15. The Part B was prepared by mixing the amounts byweight of the above Compositions I-IV, e.g., Example 168 has 4.5 gramsof Composition II and 0 grams of Composition I. The amounts of Part Bwere adjusted to maintain an OH-Amine/Epoxy ratio of about 0.8, and the% by weight of each toughener (A, B, C) based on the weight of totalepoxy is also shown. At increasing concentrations of toughener A, thePart B may thicken with time. Therefore, it may be desirable for theEpoxy composition to be mixed and applied to the substrates soon afterthe Part B is formed. The adhesives were tested for T-peel adhesion at23° C. and Impact Peel Resistance Energy at 90° C. on steel substrates,as described above. The test results are shown in Table 15.

EXAMPLES 169A, 170A, 172A-174A, 177A-182A

Part B compositions these examples were prepared in the same manner asfor Examples 169-182 except the relative amounts of part B compositionsI-IV used in Examples 169-182 were adjusted to reflect an OH/epoxystoichiometry of 0.75. The same part A composition used for examples169-182 was used.

Epoxy adhesive compositions were prepared by mixing 1.0 grams of thepart A with respective amounts of the part B compositions I-IV shown inTable 15. The adhesives were tested for Impact Peel Resistance at −40°C., −30° C. and −20° C. on steel substrates as described above. Testresults are shown in Table 15.

TABLE 15 Part B Composition Energy (J) Grams % Toughener T-peel @ Ex III III IV A B C N-cm⁻¹ 90° C. −20° C. −30° C. −40° C. 168 0 4.50 0 020.3 0 0 79 14 — — — 169 1.08 3.26 0 0 15.2 0 0 82 14 — — — 169A 1.163.48 0 0 15.2 0 0 — — 6 4 3 170 2.11 2.11 0 0 10.2 0 0 70 10 — — — 170A2.25 2.25 0 0 10.2 0 0 — — 6 4 3 171 0 0 4.61 0 0 20.3 0 117 17 — — —172 1.10 0 3.32 0 0 15.2 0 114 17 — — — 172A 1.18 0 3.54 0 0 15.2 0 — —7 7 4 173 2.12 0 2.12 0 0 10.2 0 114 17 — — — 173A 2.27 0 2.27 0 0 10.20 — — 8 5 3 174 0 0 0 4.50 0 0 20.3 158 18 — — — 174A 0 0 0 4.8 0 0 20.3— — 11 4 3 175 1.09 0 0 3.26 0 0 15.2 128 15 — — — 176 2.11 0 0 2.11 0 010.2 114 15 — — — 177 0 3.62 0.90 0 16.2 4.1 0 105 18 — — — 177A 0 3.86.96 0 16.2 4.1 0 — — 9 8 6 178 0 2.72 1.82 0 12.2 8.1 0 110 16 — — —178A 0 2.90 1.94 0 12.2 8.1 0 — — 10 9 7 179 1.72 1.29 1.29 0 6.1 6.1 0114 20 — — — 179A 1.83 1.37 1.37 0 6.1 6.1 0 — — 8 7 6 180 0 2.27 2.27 010.2 10.2 0 114 20 — — — 180A 0 2.43 2.43 0 10.2 10.2 0 — — 11 9 6 181 01.82 2.74 0 8.1 12.2 0 114 20 — — — 181A 0 1.95 2.92 0 8.1 12.2 0 — — 98 6 182 0 0.92 3.66 0 4.1 16.2 0 123 NT — — — 182A 0 .98 3.91 0 4.1 16.20 — — 8 NT 6 183 0 3.60 0 0.90 16.2 0 4.1 96 NT — — — 184 0 2.70 0 1.8012.2 0 8.1 105 15 — — — 185 1.70 1.28 0 1.28 6.1 0 6.1 114 14 — — — 1860 2.25 0 2.25 10.2 0 10.2 119 17 — — — 187 0 1.80 0 2.70 8.1 0 12.2 11718 — — — 188 0 0.90 0 3.60 4.1 0 16.2 131 16 — — — 189 0 0 3.66 0.92 016.2 4.1 131 18 — — — 190 0 0 2.74 1.82 0 12.2 8.1 140 20 — — — 191 1.720 1.29 1.29 0 6.1 6.1 128 19 — — — 192 0 0 2.28 2.28 0 10.2 10.2 152 18— — — 193 0 0 1.82 2.72 0 8.1 12.2 140 18 — — — 194 0 0 0.90 3.62 0 4.116.2 140 18 — — — NT = Not Tested

The data in Table 15 show how varying amounts, types and combinations oftougheners, with the same Part A composition, can affect T-Peel Adhesionand Impact Peel Resistance.

EXAMPLES 195-199

Part B compositions were prepared as follows:

Composition V

An epoxy resin premix composition was prepared by mixing 436 grams ofEpon 828 epoxy resin with 68 grams of EXL2600 copolymer using a highshear mixer at a temperature between 110° C. to 120° C. for about 30minutes at which time cooling was initiated while mixing continued. Whenthe temperature reached 100° C., 100 grams of catechol was added withcontinued mixing. After 5 minutes a homogeneous mix was obtained with noevidence of gel particles or undissolved catechol. Composition V wasprepared by transferring 550 grams of this premix to a planetary mixerbowl along with 65 grams of MK107 reactive diluent, 155 grams of Epon58006 epoxy resin, 199 grams of GP7I silica, 17 grams of TS-720 fumedsilica, 2 grams of K37 glass bubbles, and 13 grams of glass beads andmixing under vacuum for 20 minutes to a smooth paste like consistency.

Composition VI

An epoxy resin premix composition was prepared by mixing 470 grams ofEpon 828 epoxy resin with 46 grams of Paraloid EXL2600 copolymer using ahigh shear mixer at a temperature between 110° C. to 120° C. for about30 minutes at which time cooling was initiated while mixing continued.When the temperature reached 100° C., 46 grams of Toughener A(polytetramethylene oxide diamine) were added in approximately 5 gramamounts over a period of about 10 minutes with continuous mixing toobtain a homogeneous solution with no evidence of undissolved ToughenerA. Then 100 grams of catechol were added with continuous mixing foranother 5 minutes or until a homogenous solution was obtained.Composition VI was prepared by transferring 601 grams of this premixinto a planetary mixer bowl along with 65 grams of MK107 reactivediluent, 103 grams of Epon 58006 epoxy resin, 199 grams of GP7I silica,17 grams of TS720 fumed silica, 2 grams of K37 glass bubbles, and 13grams of glass beads and mixing under vacuum for 20 minutes to a smoothpaste like consistency.

Composition VII

An epoxy/core shell premix was prepared by mixing 560 grams of Epon 828epoxy resin with 140 grams of EXL 2600 copolymer using a high shearmixer at a temperature between 110° C. to 120° C. for about 30 minutes.676 grams of this premix was then transferred to a planetary mixingbowl. To this was added 171 grams of 58006, 71 grams of MK107, 219 gramsof GP7I, 19 grams of TS720 and 14 grams of glass beads then mixing undervacuum for 20 minutes to a smooth paste like consistency.

Composition VIII

An epoxy resin premix composition was prepared by mixing 518 grams ofEpon 828 epoxy resin with 50 grams of Paraloid EXL2600 copolymer using ahigh shear mixer at a temperature between 110° C. to 120° C. for about30 minutes at which time cooling was initiated while mixing continued.When the temperature reached 100° C., 50 grams of Toughener A was addedin approximately 5 gram amounts over a period of about 10 minutes withcontinuous mixing until achieving a homogeneous solution showing noevidence of undissolved Toughener A. Composition VIII was prepared bytransferring 562 grams of this premix to a planetary mixer bowl alongwith 71 grams of MK107 reactive diluent, 114 grams of Epon 58006 resin,219 grams of GP7I silica, 19 grams of Cab-O-Sil TS720 fumed silica, and14 grams of glass beads and mixing under vacuum for 20 minutes to asmooth paste like consistency.

Composition IX

A Part B composition was made by combining 197 grams of Epon 828, 72grams of MK107, 317 grams of the epoxy/core-shell premix described forComposition VII, 159 grams of Epon 58006 resin, 222 grams of GP7I, 19grams of TS720 and 14 grams of glass beads. This combination was mixedunder vacuum for 20 minutes to a smooth paste like consistency.

Part A compositions were prepared as follows:

Composition AA

A premix composition was made by combining 509 grams of2-ethyl-4-methylimidazoline and 254 grams of catechol in a glass jar,flushing with N2 then heating at 121° C. with occasional agitation toform a homogeneous solution. Composition AA (Part A) was prepared bytransferring 695 grams of the premix to a planetary mixer bowl alongwith 265 grams of GP7I silica, 51 grams of Cab-O-Sil TS720 fumed, and 10grams of K37 glass bubbles. These components were mixed for 5 minutes,then 10 grams of Printex 3 carbon black were added and the entirecomposition mixed under vacuum for 20 minutes.

Composition BB

A premix composition was made by combining 250 grams of2-ethyl-4-methylimidazoline, 132 grams of 3-ethoxyproplyamine, 391 gramsof catechol, and 108 grams of resorcinol in a glass jar, flushing withnitrogen, and then heating at 121° C. with occasional agitation to forma homogeneous solution. The resorcinol is added to insure that thecatechol will not recrystallize in the Part A. Composition BB (Part A)was prepared by transferring 800 grams of the premix to a planetarymixing bowl along with 156 grams of GP7I silica, 22 grams of Cab-O-SilTS720 fumed silica, and 18 grams of K37 glass bubbles and mixed for 5minutes. Then 4.5 grams of Printex 3 carbon black were added and mixedunder vacuum for 20 minutes.

Composition CC

A premix composition was prepared by combining 238 grams of2-ethyl-4-methylimidazoline, 143 grams of 3-amino-1-propanol, 376 gramsof catechol and 43 grams of resorcinol in a glass jar, flushing with N2then heating a@ 121C with occasional agitation to form a homogeneoussolution. The resorcinol is added to at least help insure that thecatechol will not recrystallize in the Part A. Composition CC (Part A)was then prepared by transferring 727 grams of the above premix into aplanetary mixing bowl along with 208 grams of GP7I silica, 13 grams ofCab-O-Sil TS720, 48 grams of K37 glass bubbles, and 4 grams of Printex 3carbon black. This combination was then mixed for 20 minutes undervacumn. The resulting catechol/3-aminopropanol weight ratio is 70/30.

Two-part adhesive compositions were prepared by mixing variouscombinations and amounts of the above Part B compositions (CompositionsV, VI, VII, VIII and IX) with the above Part A compositions(Compositions AA, BB and CC) as shown in Table 16. The amounts are givenin grams by weight, and the volumetric mix ratio of Part B to Part A(Mix Ratio—B/A). The adhesives were tested for T-Peel Adhesion andImpact Peel Resistance at 23° C. on steel, as described above. Otherdimensions of the test samples remained as described above. The catalystlevel was maintained at 4% (by weight of the total two-part adhesivecomposition). The effect of the high amine equivalent weight ofToughener A material on stoichiometry is insignificant and is notincluded in the calculations. The OH-Amine/Epoxy ratio was kept constantat about 0.8 respectively. Test results are shown in Table 16.

TABLE 16 T-Peel Energy Part A Part B Mix Ratio (N-cm⁻¹) (J) Ex GramsGrams B/A @ 23° C. @ 23° C. 195 1.08 - AA 11.61 - V 10:1  161 NT 1961.08 - AA 11.61 - VI 10:1  140 17 197 2.25 - BB 10.53 - VII 4:1 172 NT198 2.25 - BB 10.53 - VIII 4:1 145 19 199 2.00 - CC  9.02 - IX 4:1 140NT NT = Not Tested

EXAMPLES 200-231

A Part B composition was prepared as for Examples 52-58. Each catalystcomposition (Part A) was prepared by adding 47 grams of catechol, 47grams of dodecylamine, and varying amounts of imidazole or2-ethyl-4-methylimidazoline or combinations thereof as the catalyst(Cat—grams) shown in Table 17 to a jar, flushing with nitrogen, and thencapping the jar. The jars of compositions were then heated in an oven at121° C. with occasional agitation to form a Part A. The Part A was thencooled to ambient temperature.

An epoxy adhesive composition was prepared by mixing 5.0 grams of Part Bwith varying amounts of Part A shown in Table 17. The specific amount ofcatalyst in grams (Cat gms) is shown as well as the specific amount ofPart A (Part A gms), the amount of catalyst as a percent of reactivespecies, i.e. epoxy, catechol, amine, and catalyst (% Cat) and theamount of catalyst as a percent of the total adhesive formulation (%T).The OH-Amine/Epoxy ratio was 0.75 for Examples 200-215 and 0.95 forExamples 216-231. The adhesive compositions were tested for T-PeelAdhesion and Overlap Sear strength at 23° C. on lubricated steel usingG60HDES galvanized steel and a Fuchs 4107S lubricant at a coating weightof 300 mg-ft⁻².

TABLE 17 Imidazole 2-Ethyl-4-methylimidazoline Cat Part A % Cat Part AShear T-Peel Ex % Cat % T gms Gms Cat % T gms gms MPa N-cm⁻¹ 200 0.0750.05 0.32 0.88 0 0 0 0 2.5 2 201 0.148 0.10 0.63 0.88 0 0 0 0 3.3 2 2020.223 0.15 0.95 0.88 0 0 0 0 3.2 2 203 0.303 0.20 1.30 0.88 0 0 0 0 8.19 204 0.732 0.50 3.20 0.88 0 0 0 0 19.4 102 205 0 0 0 0 1.49 1.0 6.600.90 18.9 67 206 0 0 0 0 2.96 2.0 13.80 0.92 20.1 105 207 0 0 0 0 4.443.0 21.80 0.95 20.2 131 208 0 0 0 0 5.91 4.0 30.70 0.97 19.4 140 2090.043 0.022 0.15 0.92 2.96 2.0 13.80 0.92 20.1 123 210 0.084 0.042 0.290.92 2.95 2.0 13.80 0.92 19.5 123 211 0.171 0.085 0.59 0.92 2.94 2.013.80 0.92 20.0 140 212 0.338 0.228 1.60 0.92 2.92 2.0 13.80 0.92 19.9131 213 0.020 0.020 0.16 0.97 5.90 4.0 30.70 0.97 20.1 140 214 0.0410.040 0.31 0.97 5.88 4.0 30.70 0.97 20.0 123 215 0.082 0.080 0.62 0.975.88 4.0 30.70 0.97 19.9 123 216 0.073 0.05 0.26 1.11 0 0 0 0 3.3 2 2170.146 0.10 0.52 1.11 0 0 0 0 5.1 2 218 0.219 0.15 0.78 1.11 0 0 0 0 18.188 219 0.294 0.20 1.05 1.11 0 0 0 0 18.3 88 220 0.743 0.50 2.70 1.11 0 00 0 19.6 114 221 0 0 0 0 1.47 1.0 5.40 1.14 18.8 96 222 0 0 0 0 2.92 2.011.20 1.16 20.1 131 223 0 0 0 0 4.37 3.0 17.50 1.18 19.9 110 224 0 0 0 05.80 4.0 24.30 1.21 11.3 9 225 0.043 0.021 0.11 1.16 2.92 2.0 11.20 1.1619.9 78.8 226 0.087 0.041 0.23 1.16 2.92 2.0 11.20 1.16 19.9 70.1 2270.174 0.082 0.46 1.16 2.92 2.0 11.20 1.16 20 131 228 0.347 0.238 1.351.16 2.92 2.0 11.30 1.16 19.9 93 229 0.021 0.022 0.13 1.21 5.79 4.024.30 1.21 NT 2 230 0.041 0.044 0.25 1.21 5.81 4.0 24.40 1.21 NT 2 2310.082 0.087 0.50 1.21 5.81 4.0 24.50 1.21 NT 2 NT = Not Tested

The data in Table 17 show various combinations of imidazole and2-ethyl-4-methylimidazoline as well as individual catalysts alone for astoichiometry (i.e., OH-Amine/Epoxy ratio) of 0.75 and 0.95. The data inTable 17 indicates that when a combination of catalysts are used, lowerconcentrations of each catalyst can be used to provide excellent OverlapShear strength and T-Peel adhesion. In addition, by using a combinationof catalysts at an appropriate level, the performance of the adhesivecomposition (e.g., Overlap Shear strength and T-Peel adhesion) can bemaintained over a broad stoichiometric range, thereby reducing theoff-ratio sensitivity of the composition. For example, compare thedifference between Examples 208 and 224 with the similarity betweenExamples 211 and 227. Furthermore, Table 17 indicates that as theOH-Amine/Epoxy ratio increases, the amount of2-ethyl-4-methylimidazoline in the Part A can have more of an affect onthe T-peel adhesion and Overlap Shear strength (i.e., the T-peeladhesion and Overlap Shear strength become more sensitive to the2-ethyl-4-methylimidazoline concentration at higher stoichiometricratios).

EXAMPLES 232-247

Part A catalyst compositions were prepared as in Examples 94-132 bymixing catechol with various types of amines at weight ratios of 60/40and 40/60 catechol to amine (Cate/Amine Ratio) and varying amounts of2-ethyl-4-methylimidazoline as the catalyst as shown in Table 18. Theamount of Part A shown in Table 18 below was mixed with about 5 grams ofthe Part B described in Examples 52-58. The OH-Amine/Epoxy ratio wasmaintained at about 0.8. The amount of catalyst based on the totalformulation (% T) and the amount of catalyst based on the reactivespecies (% Cat) are shown with sustained load results in Table 18.

EXAMPLE 248

The catalyst (2-ethyl-4-methylimidazoline) was added directly to 5.8grams of the Part B described in Examples 1-20. Sustained load testresults are shown in Table 18.

EXAMPLE 249

A Part A composition was made by combining 4.6 grams of2-ethyl-4-methylimidazoline, 4.1 grams of catechol and 0.5 grams ofresorcinol in a jar, flushing with nitrogen, then capping the jar. Thecomposition was heated in an oven at 121° C. with occasional agitation.After a translucent, homogeneous solution was formed the jar was cooledand 0.2 grams of Printex 3 carbon black was added and mixed with thecomposition to a paste like consistency.

A Part B composition was made by combining the following ingredients ina cup and hand mixing with a wooden tongue depressor to a paste-likeconsistency: 46.9 grams of Epon 828, 7.5 grams of MK107, 6.6 grams ofthe epoxy resin premix composition described in Examples 52-58, 16.5grams of Epon™ 58006 resin, 23.1 grams of GP7I silica, 2.0 gramsCab-O-Sil™ TS 720 silica, and 0.8 grams of K37 glass bubbles. To thismixture was added 8.5 grams of catechol and uniformly distributed. Thecup was then capped and heated in the oven at 125° C. with occasionalstirring until the catechol was completely dissolved and the compositionwas mixed to a uniform consistency. The mixture was allowed to cool toambient temperature. An epoxy adhesive was prepared by combining 5.7grams of Part B with 0.5 grams of the Part A in a small ointment can andmixed to a uniform consistency. Sustained load test results are shown inTable 18.

EXAMPLE 250

Example 250 was prepared as Example 249 except that imidazole was usedas the catalyst and the stoichiometry was 0.92. Comparative ExamplesC22-C26 are commercially available and industry accepted one- ortwo-part epoxy or acrylate adhesives for bonding automotiveapplications. Comparative Example C22 is a structural one-part epoxyadhesive manufactured for Chrysler Corporation under the name MSCD 457Bby Cemedine, U.S.A. Inc. of Oak Creek, Wis., C23 is a structuralone-part epoxy adhesive manufactured for Chrysler Corporation under thename MSCD 457C by PPG Industries of Adrian, Mich., and C24 is astructural one-part epoxy structural adhesive manufactured for GeneralMotors under the name 998-1989 by PPG, C25 is an two-part acrylicstructural adhesive manufactured by Lord Corporation of Erie, Pa., underthe name Versilock™ 262.

TABLE 18 Cate/amine Cat Part A % SLD Ex Amine ratio Gms gms % T Catcycles 232 3-Amino-1-propanol 60/40 33.7 0.77 3.30 4.87 45 2333-Amino-1-propanol 40/60 36.1 0.73 3.30 4.89 24 234 Ethanolamine 60/4037 0.71 3.30 4.88 40 235 Ethanolamine 40/60 41 0.65 3.30 4.90 19 2362-Ethylhexylamine 60/40 20.1 0.90 2.50 3.66 15 237 2-Ethylhexylamine40/60 19.5 0.93 2.50 3.64 5 238 2-Ethylhexylamine 40/60 28.7 0.95 3.505.10 33 239 2-(2-Aminoethoxy)ethanol 60/40 21.5 0.84 2.50 3.67 15 2402-(2-Aminoethoxy)ethanol 40/60 21.7 0.84 2.50 3.67 4 2412-(2-Aminoethoxy)ethanol 40/60 31.9 0.87 3.50 5.13 10 242 Tridecylamine60/40 17.8 1.01 2.50 3.61 20 243 Tridecylaminc 40/60 16.2 1.12 2.50 3.604 244 Tridecylamine 40/60 23.8 1.14 3.50 5.03 34 2453-(1-Methylethoxy)-1- 60/40 20.3 0.89 2.50 3.65 6 propylamine 2463-(1-Methylethoxy)-1- 40/60 20.0 0.91 2.50 3.67 4 propylamine 2473-(1-Methylethoxy)-1- 40/60 29.3 0.73 3.50 5.11 5 propylamine 248 Noamine NA 0.20 0.20 3.30 4.90 43 249 No amine NA 0.47 0.47 3.75 5.61 30+250 3-Amino-1-propanol 52/39 9.0 0.77 1.20 1.74 34 C22 One-part Epoxy NANA NA NA NA 5 C23 One-part Epoxy NA NA NA NA NA 4 C24 One-part Epoxy NANA NA NA NA 5 C25 Two-part Acrylate NA NA NA NA NA 1 +Test wasinterrupted by metal failure of test coupon.

The data in Table 18 shows how the type and amount of amine and catalystinfluence the durability of the cured adhesive under Sustained Loadcycling conditions. In general, as the amount of amine increases,relative to the amount of catechol, a higher catalyst level may benecessary to achieve extended sustained load durability performance.

The epoxy resin adhesive composition from Example 248 was tested forSustained Load (SLD Cycles) with the different steel and lubecombinations shown in Table 19.

TABLE 19 Lube Coating Weight SLD Ex Steel Type (g-m⁻²) cycles 248G60HDES 61MAL 2.70   35 248 G60HDES Chempet BW 4.84   45+  248 A40Galvannealed* 61MAL 4.31   65+  248 A40 Galvannealed* Chempet BW 4.84  65+  *Available from National Steel +Test terminated before failure

The data in Table 19 illustrate how the type of metal, the type of lubeand the lube coating weight can influence the sustained load durability.

EXAMPLES 251-257

A part B composition was prepared as in Examples 52-58. A number of partA catalyst positions were prepared by mixing in jars 100 grams of eachof a variety of amines with varying amounts of2-ethyl-4-methylimidazoline catalyst (Catalyst Grams) as shown in Table20. Only an amine was used as a chain extender in these examples. Theamount of Part A shown in Table 20 (Part A—Grams) was mixed with 5.0grams of the Part B described above. The Amine/Epoxy ratio wasmaintained at 0.82 and the amount of catalyst (% Cat.) is shown based onweight of reactive species. T-Peel Adhesion results at 23° C. on MEKcleaned and 61MAL lubed steel are shown in Table 20.

TABLE 20 T-Peel Catalyst Part A % N-cm⁻¹ T-Peel N-cm⁻¹ Ex Amine GramsGrams Cat MEK 61MAL 251 3-Amino-1-propanol 40.8 0.66 5.14 137 70 252Benzylamine 29.7 0.87 5.07 145 88 253 2-Ethylhexylamine 25.3 1.00 4.97137 61 254 Ethanolamine 49.3 0.57 5.19 128 75 2552-(2-Aminoethoxy)ethanol 30.2 0.85 5.04 119 123 256 Tridecylamine 17.71.44 4.81 102 61 257 3-(1-Methylethoxy)-1- 26.2 0.97 5.00 131 70propylamine C26 Dodecylamine 18.7 1.35 4.82 R R R = Part Arecrystallized - not tested

The data of Table 20 shows that satisfactory results may be obtainedwith a chain extender containing only an amine. In general, as the aminecontent of the chain extender increases, the ability of the adhesivecomposition to bond to lubricated surfaces may decrease.

In the assembly of structures obtaining integrity from the adhesive ofthe invention, often a structural or frame member is combined with apanel member. Maintaining the frame and panel in a correct alignment orposition until the adhesive cures is often a difficult task. Often partsare clamped or spot welded to maintain the location of the parts. Theseassembly techniques are often not suitable for finished panels requiringsmooth uninterrupted surfaces free of dimples or any other surfacedefects. A self-aligning or positioning feature has been developed thatcan be introduced into a frame member and into a corresponding panelmember that fixes the panel member in place on the frame and maintainsits position enabling the adhesive to cure and form a strong structuralbond. Such a structure can be seen in the assembly 60 shown in FIG. 6.Frame members 61 a and 61 b are shown. These members 61 a and 61 b canbe separate, attached to other components or can be a unitary part in aframe assembly 61. A panel 62, having two or more flanges 62 a and 62 b,is fixed in place on frame members 61 a, 61 b. When made of metal (e.g.,steel), it is desirable for the frame members 61 a and 61 b to behydroformed and panel 62 to be stamped using conventional techniques. Anadhesive mass 63 is positioned between the panel 62 and each of theframe members 61 a and 61 b. The adhesive mass 63 can be one of thosedisclosed herein or any other structural adhesive. The dimensions of thepanel and members are fixed such that a portion 66 of the adhesive mass63 is wicked into the narrow separation between each panel flange 62 aand 62 b and the corresponding frame members 61 a and 61 b. The positionof the panel 62 with respect to the frame 61 is maintained bypositioning means. The positioning means keeps the panel 62 and theframe 61 in position at least long enough for the adhesive mass 63 toset or cure. For the adhesives disclosed herein, curing of this mass 63may take up to four hours or longer. The positioning means can comprisea pressure or friction fit between the panel flanges 62 a and 62 b andcorresponding frame members 61 a and 61 b. The positioning means canalso comprise one or more or a combination of channels, dimples or otherindentations 64 on one or more of the frame members 61 a and 61 b withone or more or a combination of mating ridges, bumps or otherprotuberances 65 on the corresponding panel flange 62 a and 62 b. It isalso envisioned that the indentations 64 could be on the panel 62 andthe mating protuberances 65 on the frame 61. Alternatively, one or moreof the indentations 64 and protuberances 65 could be on both the panel62 and frame 61. Furthermore, any conventional snap-fit typeconstruction may be suitable for keeping the panel 62 and frame 61together. A combination of both indentations/protuberances and pressureor friction fitting may also be desirable. Stamping or hydroforming, asapplicable, can enable these indentations 64 and protuberances 65 to beformed as the panel 62 and frame 61 are being formed. It is desirablefor the dimensions of the frame 61 and the panel 62 to be selected suchthat the adhesive positions indicated by reference numbers 63 and 66 aredimensioned such that sufficient adhesive can be used to maintain ajoint of structural integrity without wasting adhesive. Further, it isdesirable for frame 61 and panel 62 to be dimensioned so that theadhesive is rapidly wicked into place in one or, preferably, both of thejoint areas indicated by reference numbers 63 and 66. In forming theframe/panel assembly 60, the adhesive can be applied either to the panel62 or the frame member 61, or to both just prior to assembly. After theadhesive is applied, the panel 62 is simply installed, and thepositioning means locates the panel in the correct position withoutsubstantial effort. The resiliency of the material (e.g., metal) used tomake the panel 62 can be adjusted so as to allow the panel flanges 62 aand 62 b to flex and the protuberances 65 to snap in place into thecorresponding indentations 64 of the frame members 61 a and 61 b. Thisself-positioning feature places the panel 62 in the correct alignmentand creates the correct geometry for the adhesive bonds 63 and 66.Whether or not there are indentations 64 and protuberances 65 present,the resiliency of the panel material can also be adjusted so as to allowthe flanges 62 a and 62 b to flex and exert pressure against thecorresponding frame members 61 a and 61 b.

Since many embodiments of the invention can be made without departingfrom the spirit and scope of the invention, the invention resides in theclaims hereinafter appended.

We claim:
 1. A method of assembling a structure comprising: forming atwo-part epoxy composition comprising (a) a part A comprising a catalystand a chain extender, wherein the chain extender comprises a phenoliccompound, an amine compound, or a combination thereof; and (b) a part Bcomprising a reactive epoxy resin and a polymeric toughener, wherein theepoxy composition has a stoichiometric equivalent ratio of reactivehydrogen sites to reactive epoxy sites of about 0.5 to less than 1.0;applying part A and part B of the epoxy composition to at least one of afirst member and a second member (iv) sandwiching the epoxy compositionbetween the first member and the second member; and forming an adhesivebond so as to adhere the first member and the second member together,the adhesive bond comprising a thermally cured mass formed from theepoxy composition.
 2. The method of claim 1, wherein the catalyst is abasic catalyst.
 3. The method of claim 1, wherein the chain extendercomprises a compound having two reactive hydrogens.
 4. The method ofclaim 1, wherein the amine compound comprises a primary mono-amine, asecondary diamine, or a combination thereof.
 5. The method of claim 1,wherein the epoxy composition is substantially free of a polyfunctionalcuring agent.
 6. The method of claim 1, wherein the thermally cured massis cross-linked.
 7. The method of claim 1, wherein the first member is aframe member and the second member is a sheet-like member.
 8. The methodof claim 1, further comprising welding the first member to the secondmember through the adhesive bond.
 9. The method of claim 1, wherein thefirst member is a frame member and the second member is another framemember.
 10. The method of claim 1, wherein the first member and thesecond member comprises at least a portion of a vehicle.
 11. The methodof claim 1, wherein the chain extender comprises a dihydroxy phenol. 12.The method of claim 11, wherein the dihydroxy phenol comprises abisphenol, a dihyroxynaphthalene, or a dihydroxy benzene.
 13. The methodof claim 11, wherein the dihydroxy phenol is catechol or resorcinol. 14.The method of claim 11, wherein the dihydroxy phenolic comprisescatechol, 3-methoxycatechol, 3-methylcatechol, 3-fluorocatechol,4-methylcatechol, or blends thereof.
 15. The method of claim 1, whereinthe reactive epoxy resin comprises at least one of an aromatic glycidylether epoxy compound and an aliphatic glycidyl ether epoxy compound. 16.The method of claim 1, wherein the catalyst comprises a cyclic amidine.17. The method of claim 1, wherein the catalyst comprises one or moreimidazole compounds and one or more imidazoline compounds.
 18. Themethod of claim 1, wherein Part B further comprises a chain extender.19. The method of claim 18, wherein the chain extender and the epoxyresin are substantially unreacted before Part A is mixed with Part B.20. The method of claim 1, wherein the catalyst comprises a cyclicamidine and the chain extender comprises a catechol.
 21. The method ofclaim 1, wherein the catalyst comprises an imidazoline, a1,4,5,6-tetrahydropyrimidine, or a combination thereof.
 22. The methodof claim 1, further comprising welding the first member to the secondmember.
 23. A method of assembling a structure comprising: forming anepoxy composition comprising (a) catalyst comprising an imidazoline, a1,4,5,6-tetrahydropyrimidine, or a combination thereof; (b) a chainextender; (c) a reactive epoxy resin; and (d) a polymeric toughener,wherein the epoxy composition has a stoichiometric equivalent ratio ofreactive hydrogen sites to reactive epoxy sites of about 0.5 to lessthan 1.0; applying the epoxy composition to at least one of a firstmember and a second member sandwiching the epoxy composition between thefirst member and the second member; and forming an adhesive bond so asto adhere the first member and the second member together, the adhesivebond comprising a thermally cured mass formed from the epoxycomposition.
 24. The method of claim 23, wherein the chain extendercomprises a phenolic compound, an amine compound, or a combinationthereof.
 25. The method of claim 23, wherein the chain extendercomprises a dihydroxy phenol.
 26. The method of claim 23, wherein thedihydroxy phenol comprises a bisphenol, a dihyroxynaphthalene, or adihydroxy benzene.
 27. The method of claim 25, wherein the dihydroxyphenol is catechol or resorcinol.
 28. The method of claim 25, whereinthe dihydroxy phenol comprises catechol, 3-methoxycatechol,3-methylcatechol, 3-fluorocatechol, 4-methylcatechol, or blends thereof.